A  -S.  "L 


Qf. 
3 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 

ULYSSES  s.  GRA"NT 

COLLECTION 


The  RALPH  D.  REED  LIBRARY 

DEPARTMENT  OF  GBOLOGT 

UNIVERSITY  of  CALIFORNIA 

LOS  ANGELES,  CALIF. 


TWENTIETH   CENTURY  TEXT-BOOKS 


A.    F.   NIGHTINGALE,    PH.D.,    LL.D. 

FORMERLY    SUPERINTENDENT    OF    HIGH    SCHOOLS,    CHICAGO 


TWENTIETH    CENTURY   TEXT-BOOKS 


A  TEXT-BOOK  OF 

GEOLOGY 


BY 

ALBERT  PERRY  BRIGHAM,  A.M.,  F.  G.  S.  A. 

PROFESSOR   OF   GEOLOGY    IN   COLGATE    UNIVERSITY 


NEW    YORK 

D.    APPLETON    AND    COMPANY 
1903 


COPYRIGHT,  1900 
BY  D.   APPLETON  AND  COMPANY 


Geology 
Library 


PREFACE 


IN  preparing  this  volume  for  secondary  schools  the 
author  has  made  an  elementary  treatise,  and  has  avoided 
technical  discussions  and  terms  so  far  as  seemed  consistent 
with  the  purposes  of  definite  instruction.  This  is  especially 
true  in  Part  III,  whose  object  is  not  the  identification  of 
horizons  or  species,  but  to  give  a  general  understanding  of 
the  progress  of  life  and  of  the  growth  of  the  lands.  But  it 
has  not  been  thought  necessary  to  write  condescendingly  or 
in  a  juvenile  style  for  students  of  high-school  age.  The 
great  unsolved  problems  of  the  science  of  geology  have 
been  frankly  stated,  and  interested  students  will,  it  is 
hoped,  find  glimpses  of  the  vast  regions  that  lie  beyond 
the  field  of  a  brief  exposition.  Students  will  find  it  useful 
to  have  done  some  work  in  zoology  and  botany.  For  those 
who  have  not  this  preparation  for  Part  III,  the  competent 
teacher  may  give  the  supplementary  explanations  that  are 
needed.  The  order  of  treatment  is  deliberately  chosen, 
and  will  not  be  found  to  differ  in  essentials  from  that  em- 
ployed in  several  earlier  text-books.  The  phenomena  of 
weathering  and  the  various  activities  of  water  fall  at  once 
under  the  eye  of  most  students.  In  the  author's  experience 
it  has  been  found  best  to  use  at  the  outset  the  familiar  in- 
terest thus  aroused,  thus  leading  on  to  more  remote  themes. 
Other  text-books  have  been  freely  consulted,  and  record  is 
here  made  of  more  especial  indebtedness  to  Dana,  Geikie, 
and  Scott. 


GEOLOGY 

VI 

Occasional  references  to  other  works  have  been  included 
in  the  teTbut  students  desiring  a  fuller  bibliography  wrtl 
find  ft  t  the  teacher's  pamphlet  which  accompanies  the 

VOTdesire  to  record  special  obligation  to  Dr.  Charles  E. 
Boynton  and  Mr.  Kobert  E.  Cutler,  of  the  Chicago  High 
Schools     These  gentlemen  have  read  the  manuscript 
the  book,  and  have  given  many  valuable  suggestions. 

Grateful  acknowledgment  is  made  to  those  who  have 
freely  aided  in  securing  suitable  illustrations.  It  is  hoped 
that  nothing  has  been  introduced  which  does  not  truly 
illustrate  the  statements  of  the  text.  The  descriptive  titles 
have  in  many  cases  been  made  somewhat  full. 

The  following  have  given  me  cordial  assistance  :  Prof. 
H.  L.  Fairchild,  University  of  Rochester;  Prof.  George 
H.  Barton,  Massachusetts  Institute  of  Technology ;  Prof. 
William  Libbey,  Princeton  University;  Prof.  W.  H.  C. 
Pynchon,  Trinity  College;  Prof.  Samuel  Calvin,  Iowa 
State  University ;  Dr.  F.  J.  H.  Merrill,  New  York  State 
Museum;  Mr.  X.  H.  Barton,  United  States  Geological 
Survey;  Dr.  J.  M.  Clarke,  State  Paleontologist  of  New 
York,  who  has  reviewed  the  selection  of  Paleozoic  illustra- 
tions ;  Prof.  W.  M.  Davis,  Harvard  University.  Mr.  S.  E. 
Stoddard,  Mr.  W.  G.  C.  Kimball,  and  Mr.  C.  H.  James 
have  kindly  permitted  the  use  of  their  views  of  the  Adi- 
rondack region,  the  Bermuda  Islands,  and  of  Luray  Cavern. 
Many  cuts  have  been  reproduced  from  Government  and 
State  Reports,  and  from  photographs  belonging  to  the  De- 
partment of  Geology  in  Colgate  University.  In  Part  III  a 
considerable  number  have  been  taken,  with  the  author's 
consent,  from  Le  Conte's  Elements  of  Geology. 

In  the  teacher's  guide,  which  accompanies  this  volume, 
I  have  inserted  outlines  of  geological  field  excursions  for 
eighteen  of  the  greater  American  cities.  It  is  hoped  that 
these  may  be  useful  to  teachers  in  the  cities  concerned,  and 
may  serve  as  a  model  to  others.  Outlines  for  several  large 


PREFACE  vii 

centers  were  sought  but  not  secured,  hence  the  list  has 
some  serious  gaps,  such  as  St.  Louis,  New  Orleans,  Denver, 
and  San  Francisco.  Should  the  scheme  be  of  use,  this 
defect  may  be  remedied  in  future.  I  am  under  great  obli- 
gation to  those  named  below  for  the  preparation  of  the 
several  itineraries  : 

Mr.  J.  B.  Woodworth,  Harvard  University  (outline  for 
Boston) ;  Principal  David  W.  Hoyt,  English  High  School, 
Providence ;  Principal  William  Orr,  Jr.,  High  School, 
Springfield,  Mass. ;  Prof.  J.  F.  Kemp,  Columbia  University, 
Xew  York  ;  Prof.  C.  Stuart  Gager,  New  York  State  Nor- 
mal College,  Albany  (outline  for  Albany  and  Troy) ;  Prof. 
I.  P.  Bishop,  State  Normal  School,  Buffalo ;  Miss  Mary  S. 
Holmes,  Girls'  High  School,  Philadelphia ;  J.  Gordon  Og- 
den,  Ph.  D.,  Fifth  Avenue  High  School,  Pittsburg  ;  Mr. 
H.  H.  Hindshaw,  Johns  Hopkins  University,  Baltimore ; 
Prof.  H.  P.  Gushing,  Western  Reserve  University,  Cleve- 
land ;  Principal  George  W\  Harper,  Woodward  High  School, 
Cincinnati ;  Prof.  0.  C.  Lemon,  Normal  School,  Detroit ; 
Mr.  D.  C.  Ridgley,  Wrest  Division  High  School,  Chicago  ; 
Prof.  E.  C.  Case,  Normal  School,  Milwaukee  ;  Prof.  C.  W. 
Hall,  University  of  Minnesota,  Minneapolis  and  St.  Paul ; 
Mr.  George  H.  Ashley,  of  the  Indiana  Geological  Survey, 
Indianapolis ;  Major  William  J.  Davis,  Louisville.  Pro- 
fessor Kemp  was  assisted  in  preparing  the  outline  for  New 
York  by  Messrs.  Arthur  Hollick  and  G.  Van  Ingen. 

The  author  has  sought  to  treat  the  several  topics  in 
proportion  and  without  undue  bias  toward  his  own  favor- 
ite studies,  and  hopes  that  no  serious  omissions  will  be 
found. 

ALBERT  PERRY  BRIGHAM. 

COLGATE  UNIVERSITY,  October,  1900. 


CONTENTS 


PART  I 
DYNAMICAL  GEOLOGY 

CHAPTER  PAGE 

I.— GEOLOGICAL  WORK  OF  WINDS 3 

II.— WEATHERING 16 

III.— RIVERS 37 

IV.— UNDERGROUND  WATERS 76 

V.— GLACIERS 90 

VI.— LAKES 110 

VII.— THE  OCEAN 116 

VIII.— VOLCANOES 134 

IX. — MOVEMENTS  OK  THE  EARTH'S  CRUST 154 

X. — GEOLOGICAL  WORK  OK  ORGANISMS 170 

PART  II 
STRUCTURAL  GEOLOGY 

XI. — THE    ROCK-FORMING    MINERALS 191 

XII. — COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS      .        .  199 

XIII.— THE  GROSS  STRUCTURE  OF  ROCKS 215 

XIV.— PHYSIOGRAPHIC  STRUCTURES 250 

PART  III 
HISTORICAL  GEOLOGY 

XV.— GENERAL  PRINCIPLES 291 

XVI.— ARCHAEAN  AND  ALGONKIAN  ERAS  .    302 


GEOLOGY 


CHAPTER 

PAOE 

.    308 

XVII.—  PALEOZOIC  ERA. 
XVIII.—  PALEOZOIC  ERA. 
XIX—  PALEOZOIC  ERA. 
XX.—  PALEOZOIC  ERA. 
XXL—  PALEOZOIC  ERA. 
XXII.—  MESOZOIC  ERA. 
XXIII.—  MESOZOIC  ERA. 
XXIV.—  CENOZOIC  ERA. 
XXV.—  CENOZOIC  ERA. 

LOWER  SILURIAN  PERIOD 
UPPER  SILURIAN  PERIOD      . 
DEVONIAN  PERIOD  .... 
CARBONIFEROUS  PERIOD 
TRIASSIC  AND  JURASSIC  PERIODS   . 
CRETACEOUS  PERIOD 
TERTIARY  PERIOD    .... 
QUATERNARY  PERIOD 

.    318 
.    334 
.    342 
.    359 
.    378 
.    398 
.    411 
.    426 
.    465 

GE  O  L  O  GY 


INTRODUCTION 

Geology  and  its  departments. — Geology  deals  with  the 
history  of  the  earth.  It  may  have  various  subdivisions. 
For  this  text-book  we  treat  the  subject  under  the  familiar 
heads  of  dynamical,  structural,  and  historical  geology. 

Dynamical  geology  (dynamis,  force  or  energy)  treats 
of  the  forces  that  make  changes  upon  or  beneath  the  sur- 
face of  the  earth.  Hence  we  speak  of  the  work  of  the 
atmosphere,  of  water,  heat,  and  life  in  the  chapters  that 
follow. 

Structural  geology  explains  the  composition  of  rocks 
and  the  forms  in  which  they  occur.  Thus  some  rocks  are 
sandstones  or  limestones,  others  are  slates  or  schists,  and 
others  are  ancient  or  modern  lavas.  These  occur  in  beds 
or  masses  of  many  kinds,  and  have  often  been  folded, 
broken,  or  otherwise  changed.  As  a  result  of  composition, 
structure,  and  dynamic  changes  we  have  land  forms,  the 
study  of  which  is  included  in  Part  II. 

Historical  geology  takes  the  materials  of  force  and 
structure  and  builds  up  a  history  of  the  globe,  showing  the 
succession  of  its  rocks,  the  origin  of  its  lands,  and,  in  par- 
ticular, tracing  the  progress  of  living  creatures  from  remote 
ages  until  the  present  time. 

Geology  and  related  sciences. — Here  the  bond  is  close. 
Our  science  deals  with  the  earth  in  its  coming  to  be; 

l 


2  GEOLOGY 

geography  takes  up  the  earth  as  it  is,  particularly  in  its 
relation  to  man :  but  no  boundary  can  be  drawn  between 
the  two  subjects.  Geology  is  dependent  upon  chemistry 
and  physics,  for  all  rocks  have  a  chemical  constitution  and 
all  dynamical  operations  are  illustrations  of  physical  laws. 
Astronomy  aids  us  in  understanding  the  earlier  conditions 
of  the  globe.  Zoology  and  botany  contribute  largely  to 
historical  geology,  since  only  by  their  aid  can  we  know  the 
animals  and  plants  of  the  ancient  periods,  and  trace  their 
evolution  to  the  present  time. 

The  study  of  geology. — Work  in  any  branch  of  natural 
science  should  sharpen  the  observation,  quicken  the  rea- 
soning powers,  and  make  us  appreciative  of  the  world 
about  us.  Geology  offers  its  full  measure  of  these  advan- 
tages. Observation  may  deal  with  specimens  in  the  school 
collection  or  laboratory,  or  with  things  seen  in  field  excur- 
sions. Even  the  chance  walks  of  the  student  will  always  put 
geological  illustrations  before  him.  The  effort  to  construct 
conditions  found  in  other  lands  and  other  ages  will  exer- 
cise the  imagination.  Particularly  should  the  student  of 
geology  come  the  better  to  know  the  world  in  which  he 
lives ;  to  love  its  natural  scenery  because  he  understands 
it ;  to  see  meaning  in  its  rocks  and  fossils  and  in  the  mate- 
rials furnished  by  the  earth's  crust  to  the  common  arts  of 
man.  "  Whatever  frees  one  from  the  control  of  the  senses, 
or  tends  to  make  the  past,  the  distant,  or  the  future  pre- 
dominate over  the  present,  advances  us  in  the  dignity  of 
thinking  beings." 


PART    I 
DYNAMICAL     GEOLOGY 


CHAPTER  I 

GEOLOGICAL  WORK   OP   WINDS 

1.  Force  exerted. — Winds  are  movements  of  the  atmos- 
phere. Since  the  atmosphere  has  weight,  it  applies  energy, 
when  in  motion,  to  bodies  which  it  meets.  It  thus  becomes 
a  geological  agent,  which  is  more  important  than  is  com- 
monly supposed.  Winds  are  effective  according  to  their 
strength,  their  continuance,  and  the  kind  of  materials 
which  they  find  to  work  upon.  The  pressure  exerted  by 
winds  of  various  velocities  has  been  stated  as  follows  : 


Velocity  in  miles 
per  hour. 

Pressure  in  pounds 
per  square  foot. 

Light  breeze      

14 

1 

Strong  breeze              .               .... 

42 

fl 

Strong  gale 

70 

25 

Hurricane  

84 

36 

2.  Kinds  of  work  done. — Winds  attack  materials  on  or 
near  the  earth's  surface  and  accomplish  erosion,  transporta- 
tion, and  deposition. 

The  above  terms  are  so  important  throughout  the  study 
of  geology  that  their  meaning  should  be  at  once  under- 
stood. 

3 


4  GEOLOGY 

Erosion  is  the  most  general  term  for  the  gradual  destruc- 
tion of  earthy  or  rocky  masses  by  any  means,  chemical  or 
mechanical,  as  by  solution,  winds,  rivers,  or  glaciers.  The 
term  transportation  explains  itself,  but  has  special  impor- 
tance because  of  the  incessant  carriage  of  materials  through- 
out all  ages  over  all  parts  of  the  earth's  surface.  This 
will  appear  as  we  proceed.  Such  moving  materials  are 
ever  coming  to  more  or  less  enduring  rest,  in  which  they 
assume  various  topographic  forms.  To  this  phase  of  their 
history  the  term  deposition  is  applied.  Soon  or  late  it  fol- 
lows all  modes  of  transportation. 

3.  Erosion  by  winds.— Winds  can  not  commonly  lift  or 
move  rocky  masses  of  any  size  by  direct  impact ;  but  they  can 


PIG.  1. -Faceted  pebble  from  Cape  Cod,  showing  three  faces  made  and  polished 
by  blown  sand. 

pick  up  sand  grains  in  great  numbers  and  hurl  them  against 
rock  surfaces,  producing  a  large  amount  of  wear  or  abrasion. 


GEOLOGICAL  WORK  OF  WINDS  5 

Window  glass  is  among  the  hardest  of  common  sub- 
stances, but  its  surface  is  readily  ground  by  blowing  sand 
and  soon  loses  its  transparency,  as  has  been  reported  from 
exposed  dwellings  on  Cape  Cod.  Pebbles  and  bowlders  on 
the  shores  of  Cape  Cod  and  Martha's  Vineyard  and  in  the 
Androscoggin  Valley  have  thus  been  scored  by  winds  blow- 
ing in  different  directions,  forming  faces  which  meet  in 
sharp  angles.  Hence  they  have  been  termed  faceted  peb- 
bles. 

This  process  is  best  seen  upon  dry  plains  and  plateaus, 
as  in  some  parts  of  the  western  United  States,  where  winds 
are  powerful  and  vegetation  scant.  Rock  surfaces  are  pol- 
ished or  elaborately  graven  and  etched,  particularly  if  some 
layers  of  the  rock  are  harder  than  others.  As  the  sand 
moves  chiefly  near  the  ground,  bowlders  or  pillars  of  rock 
may  be  thus  undermined  until  they  topple  over,  when  the 
wear  of  the  mass  will  be  resumed  at  a  new  angle.  Orange 
trees  have  thus  been  girdled  in  California,  and  the  soft 
wood  of  telegraph  poles  cut  away,  leaving  the  knots  stand- 
ing in  relief. 

Such  work  in  nature  led  to  the  use  of  a  similar  process 
in  the  arts,  in  which  a  current  or  jet  of  angular  sand  is 
driven  against  the  surface  which  is  to  be  abraded. 

An  interesting  list  of  uses  of  the  sand  blast  may  be 
found  in  Appleton's  Popular  Science  Monthly,  September, 
1895.  Thus  the  process  is  employed  for  decorating  glass, 
for  cutting  reliefs  on  stone,  for  bringing  out  the  grain  of 
wood,  and  for  refacing  grindstones  and  emery  wheels. 

The  surface  of  the  soil  may  be  considerably  disturbed 
by  trees  falling  under  the  force  of  winds,  especially  in  the 
track  of  a  tornado.  The  hummocky  surface  of  the  forest 
grounds  is  often  due  to  this  cause.  A  disk  of  earth  ten 
feet  in  diameter  is  not  infrequently  thus  lifted.  Upon  the 
decay  of  the  supporting  roots  the  earth  settles  into  a  ridge 
with  a  pit  beside  it.  Erosion  may  thus  be  favored  by  ex- 
posing soil  to  wind  and  water,  or  be  retarded  by  obstruction 
2 


GEOLOGY 
6 


of  drainage.    Such  operations  should  not  be  thought  trivial 
t  they  are  carried  on  over  large  areas  and  throughout 

Inn 2-  periods  of  time.  . 

I  transportation  by  winds.-The  air  always  contains 
many  particles  of  matter,  which  maybe  carried  long  dis- 
tance? by  prevailing  winds.     This  is   especially   true   m 
times  of  drought,  as  maybe  seen  along  roadways  and  on 
plowed  fields.     Such  transport  is  more  important  along  the 
shores  of  the  sea  and  of  large  lakes,  where  the  sand  often 
travels  several  miles  inland  and  the  finest    dust  much 
greater  distances.     But  it  is  in  dry  regions  that  the  pro- 
cess reaches  its  height  and  has  great   geological   signifi- 
cance    Those  who  live  in  a  moist  region,  whose  soil 
held  down  by  a  network  of  vegetation,  can  not  well  under- 
stand the  severity  of  great  sand  storms  in  an  arid  country. 
Not  seldom  they  overwhelm  man  and  beast,  hide  the  sun 
from  view,  and  continue  for  many  hours.     Such  a  storm 
swept  over  some  of  the  Northwestern  States  on  May  6  and 
7, 1899,  and  is  thus  described :  "  The  air  was  filled  with 
flying  particles,  caught  up  from  the  plowed  fields,  from  the 
blackened  prairies,  from  the  public  roads,  and  from   all 
sandy  plains.     These  particles  formed  dense  clouds  and 
rendered  it  as  impossible  to  withstand  the  blast  as  it  is 
to  resist  the  '  blizzard '  which  carries  snow  in  the  winter 
over  the  same  region.     The  soil,  to  the  depth  of  four  or 
five  inches  in  some  places,  was  torn  up  and  scattered  in  all 
directions.     Drifts  of  sand  were  formed  in  favorable  places 
several  feet  high,  packed  precisely  as  snowdrifts  are." 

To  this  process  is  largely  due  the  deep  covering  of  soil 
and  sand  which  hides  the  remains  of  many  ancient  cities  in 
the  East.  Layard  and  others  have  encountered  dust  storms 
while  engaged  in  uncovering  such  ruins.  All  work  ceased, 
day  was  turned  into  night,  and  the  laborers  crouched  in  the 
trenches  to  save  themselves  from  suffocation.  Prof.  J.  A. 
Udden  reports  six  sand  storms  as  taking  place  in  Arizona 
in  1893.  Many  more  than  this  occur  in  parts  of  California. 


GEOLOGICAL   WORK  OF  WINDS  7 

The  extent  of  single  storms  varies  from  80  to  400  miles, 
and  twenty-four  hours  is  a  safe  average  for  their  duration. 
The  same  writer  estimates  that  hundreds  or  even  thou- 
sands of  tons  of  dust  are  carried  in  a  cubic  mile  of  air  at 
such  times. 

Volcanoes  are  a  great  source  of  wind-borne  dust.  This 
will  be  explained  in  a  later  section.  It  is  enough  here  to 
note  that  vast  quantities  of  fine  rocky  matter  are  expelled 
in  explosive  eruptions  and  widely  scattered  over  land  and 
sea.  The  heavier  particles  descend  in  the  vicinity,  but  the 
finer  dust  is  carried  to  great  heights  in  the  air  and  distrib- 
uted by  the  winds  over  the  entire  globe.  Such  dust  was 
seen  by  many  in  Norway  and  Sweden  on  March  29  and 
30,  1875.  It  fell  on  clothing,  gave  pain  by  lodging  in  the 
eyes,  and  was  observed  on  the  glass  covering  of  green- 
houses. It  proceeded  from  an  eruption  in  Iceland,  several 
hundred  miles  away,  and  had  been  less  than  twenty-four 
hours  in  transit. 

A  still  more  striking  and  famous  illustration  is  found 
in  connection  with  the  eruption  of  Krakatoa,  a  volcano  in 
the  East  Indies,  in  1883.  The  finer  dust  is  reported  to  have 
ascended  to  a  height  of  17  miles,  and  is  believed  to  have 
been  carried  around  the  world.  To  the  presence  of  this 
dust  is  ascribed  the  lurid  hue  of  the  skies  during  the  late 
summer  and  the  autumn  of  that  year. 

These  striking  examples,  remote  from  common  view, 
should  not  lead  us  to  neglect  the  importance  of  wind  car- 
riage as  going  on  everywhere.  Such  transportation  does 
not  compare  with  that  of  streams,  but  in  seeking  to  under- 
stand the  history  of  the  earth,  and  to  know  how  it  has  be- 
come suited  to  human  life,  we  must  neglect  nothing,  for 
small  causes  at  last  achieve  large  results.  An  English 
geologist  believes  that  the  fertility  of  English  soils  is 
largely  due  to  the  winter  and  March  winds,  especially  when 
we  remember  that  the  climate  of  glacial  times  was  colder 
and  the  soil  less  protected  by  vegetation.  He  cites  cases 


g  GEOLOGY 

of  church  towers,  even  in  marshy  regions,  upon  whose  top 
enough  soil  has  lodged  to  support  growing  plants. 

5.  Deposition  by  winds.— The  dust,  swept  over  all  conti- 
nents and  seas,  is  as  constantly  deposited  upon  land  sur- 
faces, or  is  sinking  to  the  sea  bottoms  to  mingle  with  other 
materials  there  accumulating.  But  in  favorable  situations 
much  dust  comes  to  repose  locally,  forming  hills  known  as 
sand  dunes.  These  hills  may  be  steep  in  slope  and  limited 
in  area,  or  they  may  stretch  with  gentle  undulations  over 
a  considerable  territory.  The  conditions  for  dune-making 
are  abundance  of  fine  material,  lack  of  plant  covering,  and 
strong  winds.  It  is  plain  that  these  conditions  are  often 
met  in  desert  and  shore  regions.  In  shore  belts  the  grind- 
ing of  the  waves  produces  the  material  and  prevents  plant 
growth,  while  the  winds  sweep  unhindered  over  the  waters. 
The  dryness  of  the  air  and  disintegration  of  the  rocks  effect 
similar  results  in  deserts.  The  dune  hills  may  lie  in  ridges 
with  intervening  hollows,  like  a  succession  of  waves  and 
troughs,  or  they  may  be  disposed  in  an  irregular  and  dis- 
orderly way.  They  vary  in  height  from  a  few  feet  to 
more  than  200.  The  inclination  of  their  surfaces  is  often 
gentle  on  the  windward  side,  but  may  be  quite  steep  on 
the  lee  side,  since  the  sand  falls  over  the  crest  and  conies 
to  rest  at  the  steepest  angle  which  such  loose  material  will 
assume.  The  surface  often  bears  ripple  marks,  such  as  are 
made  by  waves  in  shallow  water.  Internally,  if  a  cutting 
be  made  through  the  sands,  they  may  show  layers  or  strata 
due  to  deposition  at  different  times,  or  by  winds  of  dif- 
ferent velocities,  which  would  bring  materials  of  various 
sizes.  The  inclination  of  the  layers  may  show  great  variety, 
dependent  on  the  direction  of  the  original  slopes  which 
received  them. 

Scanty  vegetation  is  often  found  on  dune  surfaces,  and 
sometimes,  as  on  Cape  Cod,  plants  have  been  set  by  the 
hand  of  man  to  hold  down  the  sands  and  prevent  the  devas- 
tation of  cultivated  fields  lying  beyond. 


10 


GEOLOGY 


6.  Migration  of  dunes.— Xot  only  the  sand  grains, 
but  the  dunes,  may  travel  for  some  distances.  The  wind 
picks  up  particles  on  all  parts  of  the  exposed  slope  and 
carries  them  over  the  crest.  It  is  evident  that  the 
crest  and  lee  slope  will  slowly  advance  in  the  direction 
of  the  wind.  Illustrations  of  this  process  will  be  found 
in  the  following  paragraphs  upon  the  distribution  of  sand 
hills. 


FIG.  3.— Sand  dunes,  north  from  Monomoy  Lighthouse,  Cape  Cod. 

(Copyright  by  S.  R.  STODDARD,  Glens  Falls,  N.  Y.) 

7.  Sand  dunes  in  shore  regions. — Dunes  abound  along  the 
eastern  coast  of  the  United  States.  On  the  shores  of  Mas- 
sachusetts and  Long  Island  much  fine  earthy  debris  was 
deposited  by  glacial  ice.  This  is  easily  worked  over  by  the 
waves,  and  becomes  the  sport  of  the  winds.  Extensive 
dunes  are  found  north  of  Cape  Ann,  along  the  outer  shores 
of  Cape  Cod,  and  on  the  islands  of  Nantucket  and  Martha's 
Vineyard.  Headers  of  H.  D.  Thoreau's  Cape  Cod  will 
find  graphic  accounts  of  the  wind-blown  sands.  Dana 
notes  the  presence  of  sand  ridges  for  100  miles  along  the 
shore  of  Long  Island.  From  Xew  Jersey  southward  the 
lands  consist  of  marine  muds  and  sands  lifted  above  the 


GEOLOGICAL   WORK  OF  WINDS 


11 


sea  level  in  late  geological  time.  Hence  they  are  rela- 
tively loose  deposits,  readily  crumbled  by  the  waves,  and 
built  by  the  winds  into  sand  hills. 

The  Bermuda  Islands,  whose  highest  hills  have  an 
altitude  of  more  than  200  feet,  are,  from  a  few  feet  above 
the  sea  level,  eolian  formations.  Calcareous  sand,  frag- 
ments of  corals,  and  shells  ground  up  by  the  waves,  are 


Fie.  4.— Beds  of  limestone  deposited  by  wind,  shore  cliffs,  Bermuda  Islands. 
Photograph  by  W.  G.  C.  KIMBALL. 

thus  effectively  used  by  the  winds  for  the  making  of 
land.  Eain  waters  carrying  lime  in  solution  have  cement- 
ed many  of  these  sands  into  hard  rock,  which  may  be 
seen  in  quarry  walls  and  in  the  rugged  shore  cliffs  of  the 
islands. 


12  GEOLOGY 

Many  good  examples  occur  on  the  shores  of  Europe. 
Abundant  dunes  are  found  on  the  exposed  coast  of  Norfolk 
and  Cornwall  in  England.  In  the  latter  district  a  church 
was  for  seven  centuries  smothered  in  the  migrating  sand 
hill,  but  came  to  light  in  1835.  On  sections  of  the  French 
coast  the  dune  belt  has  an  average  width  of  3  miles,  and, 
according  to  Prestwich,  is  advancing  inland  30  to  60  feet 
in  a  year.  On  the  west  coast  of  France  streams  have  been 
forced  by  the  encroaching  drifts  to  wander  along  the  shore, 
seeking  an  outlet  to  the  sea.  In  Denmark  the  migration 
of  the  dunes  has  in  some  places  been  checked  by  planting 
groves  of  pine  trees.  Holland  has  a  belt  of  shore  dunes 
ranging  from  50  to  260  feet  in  height,  and  from  1  to  5  miles 
in  width.  Writers  on  the  geography  of  Palestine  describe 
the  "  endless  mounds  of  drift  sand  "  which  impede  tillage 
and  have  mantled  the  ruins  of  the  ancient  Philistine  cities 
of  the  Mediterranean. 


FIG. 

Scale,  one 


5. -Sand  dunes  south  of  Kinsley,  Kail. 
mile  to  the  inch.    Contour  interval  20  feet. 


^   Extensive  dunes  may  be  formed  along   lake   shores, 
ravelers  entering  or  leaving  Chicago  on  the  east  may  see 


GEOLOGICAL  WORK  OF   WINDS  13 

many  such  hills,  sometimes  entirely  bare  of  vegetation,  as 
at  Michigan  City.  Mammoth  dunes  are  found  in  Michigan 
on  the  eastern  shore,  at  Grand  Haven  and  other  points. 

8.  Dunes  in  arid  regions. — Wide  stretches  of  dune  coun- 
try may  be  seen  in  western  Kansas  (see  map),  Nebraska, 
Colorado,  Wyoming,  and  Arizona.     The  same  occur  in  the 
Sahara,  and  in  the  vast  plains  of  Central  Asia.    A  striking 
geographical  result  of  drifting  sand  is  the  changed  course 
of  the  river  Oxus,  which,  having  formerly  flowed  into  the 
Caspian,  now  enters  the  Sea  of  Aral.     Drifting  sands  of 
deserts  are  greater  in  amount  than  those  of  shores,  but  are 
less  conspicuous  in  their  relation  to  human  interests,  and 
have  not  been  so  widely  observed  and  described. 

9.  Loess. — In  the  Mississippi  and  Ehine  valleys,  and  in 
the  high  basins  and  on  the  plains  of  central  China,  are 
great  deposits  to  which  this  name  has  been  given.     Nearly 
all  observers  agree  that  the  winds  have  had  much  to  do 
with  the  present  character  of  these  great  sheets  of  earthy 
matter.     The  typical  loess  in  China  and  other  regions  is  a 
yellowish  or  brownish  earth  or  loam,  without  stratification, 
and  sometimes  attaining  a  thickness  of  more  than  2,000 
feet.     It  has,  however,  a  vertical  structure  of  fine  tubes, 
believed  to  be  due  to  the  roots  and  stems  of  plants  occupy- 
ing the  successive  surfaces  as  the  deposit  has  grown.     Thus 
there  arises  a  cleavage  by  which  vertical  cliffs  are  formed 
along  streams  and  highways,  dissecting  the  country  into  a 
labyrinth  of  plateaus  and  narrow  valleys.     The  peculiar 
characters  of  the  loess  lend  themselves  to  human  uses  in 
an  interesting  manner  in  China,  as  thus  described  by  Pum- 
pelly : 

"  This  remarkable  combination  of  softness  with  great 
strength  and  stability  of  exposed  surfaces  is  of  inestimable 
value  in  a  woodless  country.  In  Asia  thousands  of  villages 
are  excavated  in  the  most  systematic  manner  at  the  base  of 
cliffs  of  loess.  Doors  and  windows  pierced  through  the 
natural  front  give  light  and  air  to  suites  of  rooms,  which 


GEOLOGY 

are  separated  by  natural  walls  and  plastered  with  a  cement 
made  from  the  loess  concretions.  These  are  the  comfort, 
able  dwellings  of  many  millions  of  Chinese  farmers,  and 
correspond  to  the  rude  '  dugouts  '  of  Nebraska. 


FIG.  6.— Quarry  in  limestone  formed  of 

Photograph  by  W.  G.  C.  KIMBAI.I.. 

In  the  Mississippi  valley  the  beds  have  less  thickness, 
and  are  believed  to  be  first  due  to  flood  streams  from  the 
continental  glacier. 

These  streams  wandered  widely  over  low  grounds,  and 
deposited  fine  materials  brought  from  the  north.  These 
materials  have  since  been  worked  over  in  large  measure  by 


GEOLOGICAL  WORK  OF  WINDS  15 

winds.  Similar  deposits  are  found  in  Utah,  Nevada,  and 
California,  and  are  ascribed  primarily  to  the  transporting 
action  of  streams,  which  carry  rock  flour  from  the  surround- 
ing mountains  and  spread  it  over  the  plains. 

10.  Indirect  geological  effects  of  winds. — Here  we  observe 
(1)  the  making  of  waves  on  oceans  and  lakes,  the  waves  in 
turn  working  important  changes  along  the  shores,  as  will 
appear  in  later  chapters.  (2)  Transfer  of  water  vapor. 
Such  vapor  arising  from  bodies  of  water  overspreads  the 
lands  by  means  of  winds,  giving  rise  to  streams  of  water 
and  ice,  the  most  effective  of  geological  tools.  (3)  Transfer 
of  heat,  directly  and  by  means  of  ocean  currents,  determin- 
ing the  groups  of  animals  and  plants  which  shall  occupy 
the  lands  and  seas,  and  thus  in  turn  modifying  soil  forma- 
tion, denudation  of  land  surfaces,  and  powerfully  control- 
ling the  life  of  man.  We  have  here  a  good  illustration  of 
the  blending  of  causes  and  effects  in  the  history  of  the  earth. 
(4)  Carriage  of  seeds  and  other  organic  forms.  Many  plants 
have  seeds  or  stems  which  adapt  them  to  such  movements, 
as  the  dandelion,  or  the  Russian  thistle  infesting  the  plains  of 
the  Northwest.  Birds  are  swept  from  continents  to  islands, 
or  from  one  continent  to  another.  McCook  describes  a 
species  of  spider  which  has  thus  come  to  inhabit  the  circuit 
of  the  globe.  Darwin  many  years  ago  collected  dust  on 
shipboard,  nearly  or  quite  1,000  miles  from  the  African 
shore,  and  the  dust  was  found  to  contain  multitudes  of 
lowly  microscopic  organisms  brought  from  the  land.  We 
may  safely  say,  therefore,  that  the  indirect  geological  effects 
of  winds  are  greater  than  those  which  they  accomplish  im-. 
mediately. 


CHAPTER  II 

WEATHERING 

11.  IF  we  observe  a  ledge  of  rock  naturally  exposed  in  a 
hillside  field  or  on  the  banks  of  a  stream,  we  shall  find  evi- 
dences of  decay.  The  outer  parts  are  commonly  discolored, 
cracked,  and  may  even  crumble  under  the  pressure  of  the 
hand.  If  the  slope  is  steep,  pieces  will  have  fallen  off,  to 
form  a  heap  of  coarse  or  fine  rubbish  at  the  bottom.  In 
like  manner,  if  we  examine  rock  which  has  been  artificially 
exposed  for  some  years,  we  shall  find  the  beginnings  of 
decay,  as  in  railway  cuttings  or  upon  stone  fences  or  build- 
ings. Flakes  scale  off,  corners  become  rounded,  scars  and 
cracks  appear,  and  an  aspect  of  age  is  taken  on.  Let  us 
visit  also  a  sand  bed  or  gravel  pit.  We  shall  be  likely  to 
see  at  the  top  a  thin  layer  of  dark  soil  with  roots,  then 
more  or  less  loamy,  brown  or  yellowish  earth,  and  below, 
the  undisturbed,  often  bluish  beds  of  sand  and  gravel. 
The  upper  beds  have  suffered  changes  of  color,  constitution, 
and  form  from  which  the  lower  beds  have  been  free.  We 
may  also  study  with  profit  a  heap  of  cobble  stones  gathered 
out  of  a  field.  Some  are  hard  and  apparently  unchanged, 
or,  if  decay  has  begun,  only  the  outer  film  is  affected. 
Others  break  up  with  a  slight  blow,  or  may  be  shaven  fine 
with  a  pocket  knife,  showing  that  some  rocks  yield  more 
easily  than  others  to  destructive  agencies. 

To  the  processes  of  which  several  illustrations  have  now 

been  given,  the  comprehensive  term  weathering  is  applied. 

The  term  expresses  the  sum  total  of  changes  which  come 

to  a  rock  mass  under  exposure  at  or  slightly  below  the  sur- 

16 


WEATHERING 


17 


face  of  the  earth.  The  process  is  a  highly  composite  one. 
Air,  water,  heat,  and  other  agents  are  at  work,  and  we 
might  distribute  their  effects  under  the  great  divisions  of 
dynamical  geology.  But  the  processes  are  so  general,  so 


FIG.  7.— Magog,  Pike's  Peak  trail.    Bowlders  formed  in  place  by  weathering. 

linked  together,  and  so  important,  that  we  must  at  once 
take  a  comprehensive  survey  of  them.  The  central  fact  in 
the  earth's  history  is  that  rocks  in  all  lands  are  in  process 
of  destruction,  and  in  all  seas  are  being  made  anew.  It  is 
necessary  for  us  at  once  to  see  that  destruction  of  rocks  is 
quietly  going  on  everywhere,  and  hence  we  proceed  to  a 
fuller  account  of  weathering. 

12.  Definitions. — A  rock  in  geology  is  any  aggregate  of 
rocky  or  earthy  matter,  whether  consolidated  or  not.     As 


GEOLOGY 

lo 

thus  defined,  rock  includes  sands,  clays,  peat  beds,  and 
soils  Rocks  are  often  in  beds  or  layers,  and  are  said  to  be 
stratified.  In  undisturbed  regions  the  beds  are  nearly  hori- 
zontal Planes  of  division  often  intersect  the  beds 
nearly  vertical  direction.  These  are  called  joints.  A  wall- 
like  mass  of  rock  in  a  molten  condition  is  sometimes  in- 
truded into  a  crack  in  other  rocks.  It  may  vary  in  thickness 
from  a  fraction  of  an  inch  to  many  feet,  and  is  called  a 
dike.  A  rock  that  is  largely  composed  of  carbonate  of 
lime  is  a  limestone.  A  rock  made  up  of  broken  grains  of 
older  rocks  bound  together  by  a  cement  is  sandstone.  Fine 
flour  of  older  rock  masses  compacted  again  into  rock  is  a 
shale.  Limestones,  sandstones,  and  shales  show  endless 
variety  of  constitution,  texture,  and  color.  These,  and 
other  rocks  which  are  less  common  in  most  regions,  will 
be  described  in  Part  II. 

WEATHERING  AGENTS 

We  will  now  study  certain  substances  and  forces  in  their 
relation  to  weathering.  While  we  must  take  them  up  one 
by  one,  it  is  important  to  remember  that  several  of  them 
may  be  operative  at  the  same  time  upon  a  given  mass  of 
rock. 

13.  (1)  The  atmosphere.— Dry  air  has  little  direct  chemi- 
cal effect  upon  rocks,  though  its  oxygen  may  to  a  slight 
degree  combine  with  some  elements  of  rocks,  and  thus  by 
slow  combustion  contribute  to  their  decay.     The   nitric 
acid  and  ammonia  of  the  air  are  small  in  amount,  and, 
according  to  Merrill,  are  of  slight  account  in  weathering. 
The  carbon  dioxide  is  more  important,  and  with  the  oxygen 
becomes  an  effective  destructive  agent  in  connection  with 
water. 

14.  (2)  Water. — The  blows  inflicted  by  raindrops,  espe- 
cially in  a  violent  shower,  have  considerable  erosive  effect 
upon  unconsolidated  rocks,  such  as  soils,  sands,  and  clays. 


WEATHERING 


19 


Little  by  little  the  surface  portions  are  broken  up  and  car- 
ried away  by  the  rills  which  are  formed.  Thus  we  come  to 
the  work  of  running  water,  to  be  studied  in  the  next  chap- 
ter. The  efficiency  of  rain  is  due  to  its  constant  and 


FIG.  8.— Rain  erosion,  Garden  of  the  Gods,  Col.    Photograph  by  the  author. 

widespread  action.  We  must  remember  also  that  there 
are  regions  of  violent  and  prolonged  rain,  in  which  from 
two  to  ten  times  as  much  moisture  falls  annually  as  in 
the  northern  United  States.  Considerable  pillars  have 
been  formed  by  rain  out  of  stony  clay.  The  stones  protect 
the  clay  beneath  from  wash,  and  thus  cap  the  rude  columns 
which  are  formed  by  the  carrying  away  of  the  surrounding 
mass.  Sometimes  these  pillars  are  many  feet  in  height. 


20 


GEOLOGY 


The  water  which  exists  in  the  air  as  vapor,  and  that  which 
filters  through  the  surface  layers  of  rock  and  soil,  does  its 
geological  work  mainly  by  chemical  changes.  Among  these 
changes  solution  is  one  of  the  most  important.  Pure  water 
has  a  solvent  effect  upon  all  rocks,  even  though  it  be  slight. 
But  water  in  nature  is  never  absolutely  pure — that  is,  with- 
out other  ingredients  than  its  constituent  hydrogen  and  oxy- 
gen. It  gathers  to  itself  acids  and  gases  of  various  kinds, 
which  enable  it  slowly  to  eat  away  rocks  and  take  their  par- 


'IG.  9.-Erosion  columns  made  by  the  weathering  of 
Note  the  protecting  bowlders. 


tides  into  solution.  Nearly  everywhere  the  earth  is  covered 
with  a  mantle  of  decaying  plants.  Such  decay  causes  the 
formation  of  substances  known  as  the  humus  acids 


WEATHERING  21 

Water  soaking  through  the  soil  takes  up  these  acids  and 
thereby  dissolves  the  rocks.  Most  important  of  these  is 
carbon  dioxide,  which  is  gathered  not  only  from  the  soil, 
but  from  the  atmosphere  by  the  falling  rain.  Water  con- 
taining this  gas  dissolves  many  substances  effectively,  espe- 
cially carbonate  of  lime.  If  rocks  largely  made  up  of  this 
or  other  soluble  minerals  are  subjected  to  the  action  of 
carbonated  waters,  the  soluble  matters  are  washed  away, 
leaving  what  is  known  as  a  residual  soil.  Thick  beds  of 
rock  are  sometimes  greatly  reduced  by  this  process.  A 
number  of  illustrations  will  now  be  given  to  show  the 
geological  importance  of  solution.  Some  of  these  are 
quoted  from  Merrill. 

In  Arkansas,  analysis  of  a  sample  of  fresh  limestone  and 
of  clay  left  as  a  remnant  by  solution,  showed  that  over  97£ 
per  cent  of  the  original  rock  has  disappeared.  A  marble 
clock  case  packed  in  damp  excelsior  from  May  to  October 
required  repolishing.  Over  many  narrow  bands  where  the 
fibers  rested  the  surface  had  been  sufficiently  dissolved  to 
destroy  the  luster.  Various  minerals  digested  for  forty- 
eight  hours  in  carbonated  waters  gave  off  from  0.4  to  1  per 
cent  of  their  mass. 

If  we  now  remember  that  limestones  extend  over  wide 
areas,  and  that  acidulated  waters  are  always  flowing  over 
them  and  soaking  through  them,  we  shall  see  that  solution 
is  vastly  important  in  geology.  It  has  been  computed  that 
275  tons  of  lime  carbonate  are  annually  removed  from  each 
square  mile  of  limestone  country  in  the  Nittany  Valley  of 
central  Pennsylvania.  Another  estimate  gives  the  annual 
removal  of  all  soluble  matters  by  solution  from  the  surface 
of  England  and  Wales  as  143.5  tons  per  square  mile.  The 
meaning  of  these  facts  as  regards  the  general  reduction  of 
land  surfaces  will  be  considered  in  the  following  chapter. 
The  effect  upon  water  supply  is  one  of  the  most  important 
results  of  solution.  If  the  surface  materials  contain  much 
lime,  the  water  will  be  "  hard,"  as  it  is  termed,  encrusting 

3 


GEOLOGY 


tea  kettles,  and  will  be  unfit  for  some  domestic  uses.  Many 
regions  of  less  soluble  rooks  are  overspread  by  limestone 
flour  produced  and  transported  by  glaciers  and  readily  solu- 
ble by  surface  waters.  In  Scotland,  Loch  Katrine  lies  in  a 


Pio.  10.— Calcareous  sandstone,  showing  weathered  windy  rim  and  a  solid  core 
which  contains  the  original  carbonate  of  lime. 

region  of  less  soluble  rocks,  and  hence  furnishes  soft  water 
to  the  great  city  of  Glasgow.  The  same,  according  to  Ram- 
say, is  true  of  districts  in  northern  England,  whence  Liver- 
pool, Sheffield,  and  other  cities  are  supplied.  On  the  con- 
trary, the  waters  of  the  south  and  east  of  England,  derived 
from  the  chalk  and  other  limestone  formations,  are  very 
hard.  The  Bath  Old  Well  sends  out  enough  lime  in  its 
waters  in  one  year  to  make  304  cubic  yards  of  limestone. 

A  rock  may  consist  mainly  of  minerals  not  easy  to  dis- 
solve, cemented  together  by  a  small  amount  of  soluble  ma- 
terial. Some  sandstones  consist  chiefly  of  grains  of  quartz 
bound  together  by  a  very  small  percentage  of  lime.  The 
solution  of  the  latter  may  cause  the  entire  mass  to  crumble. 

The  oxygen  of  water  often  unites  with  materials  of  rock 
and  causes  decay.  This  process  is  known  as  oxidation. 
Oxygen  unites  very  readily  with  iron,  causing  slow  combus- 
tion, producing  "  rust,"  and  leading  to  the  disintegration 
of  the  rock  which  contains  it.  Oxidation  is  perhaps  com- 
parable in  geological  importance  to  solution. 


WEATHERING  23 

Hydration  (literally  watering)  is  also  important  in  some 
cases.  Rocks  may  take  water  into  union  with  one  or  more 
of  their  constituent  minerals,  which  are  thereby  increased 
in  bulk,  and  strains  are  set  up  by  which  the  coherence  of 
the  rock  is  injured  and  it  may  disintegrate. 


FIG.  11.— Talus  from  fallen  block,  Ouray  and  Silverton  Toll  Road,  Col. 
Photograph  by  the  author. 

15.  (3)  Heat  and  cold. — Rocks,  like  other  substances, 
expand  upon  receiving  heat  and  contract  when  it  is  with- 
drawn. The  outer  portions  of  a  rock  mass  are  subject  to 


2j.  GEOLOGY 

greater  alternations  of  heat  and  cold  than  those  which  lie 
beneath,  and  surface  parts  are  likely  to  flake  off.  Obser- 
vations have  shown  that  Bunker  Hill  Monument  is  swayed 
during  a  sunny  day  by  the  greater  expansion  of  the  heated 
side.  The  movement  is  slight,  but  in  the  course  of  cen- 
turies will  contribute  toward  the  destruction  of  the  pile. 
Dr.  Livingston  reported  from  Central  Africa  the  effective 
rending  of  rocks  which  were  raised  to  a  temperature  of 
137°  during  the  day,  with  abrupt  loss  of  heat  at  night. 
Geikie  observed  90°  and  20°  as  daily  extremes  of  tempera- 
ture in  the  Yellowstone  Park.  In  the  same  region  Merrill 
saw  fresh  chips  of  black  rock,  made  in  this  manner,  but 
bearing  much  resemblance  to  the  handiwork  of  man.  Such 
daily  variations  of  temperature  do  not  commonly  extend 
below  three  feet  from  the  surface,  but  annual  variations, 
as  between  winter  and  summer,  affect  the  rocks  at  much 
greater  depths. 

It  is  here  convenient  to  consider  the  breaking  of  rocks 
due  to  water  freezing  in  crevices,  as  along  bedding  or  joint 
planes,  or  fractures.  Freezing  water  at  a  temperature  of 
30°  exerts  a  pressure  of  138  tons  per  square  foot.  It  thus 
becomes  a  resistless  rending  agent.  At  the  brow  of  a 
cliff  blocks  may  be  frequently  found  to  be  wedged  off  from 
a  part  of  an  inch  to  a  foot  or  more.  The  opening,  narrow 
at  first,  is  filled  only  by  water,  afterward  by  water  and  soil, 
which  expand  with  freezing,  giving  the  adjoining  rocks  a 
slight  but  effective  push  which  eventually  carries  their 
center  of  gravity  beyond  the  point  of  support. 

16.  (4)  Organisms.— The  general  effects  of  living  forms 
in  modifying  the  surface  of  the  earth  will  be  set  forth  in 
a  separate  chapter.  We  must  here  briefly  notice  their 
contribution  to  the  disintegration  of  rocks.  Falling  leaves 
and  the  decaying  trunks  and  branches  of  trees  and  her- 
baceous plants  cover  the  earth  with  a  more  or  less  com- 
plete mantle  of  vegetable  mold.  Numerous  acids  are  pro- 
duced by  this  decay  and  are  contributed  to  the  rain  water 


WEATHERING  25 

which  soaks  into  the  ground,  thereby  greatly  increasing 
its  solvent  power.  Similar  effects  are  produced  by  lichens 
and  other  lowly  plants,  which  attach  themselves  to  rocks, 
keeping  their  surfaces  moist  as  well  as  supplying  destruc- 
tive acids.  The  crumbling  surface  of  stone  walls  will  offer 
abundant  illustrations. 

Mechanical  rending  of  great  importance  results  from 
the  growth  of  roots  and  trunks,  which  establish  themselves 
in  crevices  and  push  apart  the  adjacent  masses  of  rock 
with  almost  resistless  force.  Wherever  trees  grow  on  the 
edges  of  a  ravine  or  in  fields  where  rocky  ledges  protrude 
from  the  scanty  soil,  such  work  may  be  seen.  Even  young 
and  succulent  roots  are  effective  in  this  manner,  and  hence 
the  process  is  almost  universal. 

Plants  may  retard  weathering  by  interfering  with  the 
conduction  of  heat  and  by  the  formation  of  even  tempera- 
ture, and  also  by  protecting  surfaces  from  the  action  of 
winds. 

17.  (5)  Gravitation. — All  earthy  and  rocky  matter  shows 
what  we  may  call  the  downhill  tendency,  unless  supported 
by  surrounding  masses.  Energy  is  thus  applied  so  widely 
and  so  silently  that  its  immense  importance  in  geology  is 
commonly  overlooked.  From  the  crests  of  waterfalls  and 
the  borders  of  deep  cafions  great  masses  may  fall.  This 
is  more  conspicuous  but  less  important  than  the  steady 
downward  "  creep  "  of  stones  and  soil  on  all  slopes.  The 
presence  of  moisture  relieves  the  friction  between  parti- 
cles, and  a  direct  offthrust  is  made  by  freezing  water. 
Slight  slips  give  rise  to  small  benches  running  along  the 
face  of  slopes.  These  are  more  often  due,  however,  to  the 
tracking  of  sheep  or  cattle.  Under  favorable  conditions 
such  movements  are  extensive  and  sudden,  and  we  term 
them  landslips  or  landslides.  These  will  be  noticed  in  de- 
scribing the  work  of  subterranean  waters. 

Here  we  may  class  the  effects  of  avalanches  or  great  snow 
slides  in  mountain  regions.  On  the  high  slopes  of  the  San 


26  GEOLOGY 

Juan  about  Silverton,  Colorado,  open  lanes  descend  be- 
tween belts  of  forest,  being  the  track  of  avalanches.  The 
uprooting  of  trees  exposes  the  soil  to  removal  and  may 
determine  a  water  course.  A  great  number  of  such  ava- 


FIG.  12.— Angular  waste,  timber  line,  Pike's  Peak  trail. 

lanche  tracks  may  be  seen  in  the  Alps,  and  the  Swiss  people 
drive  wooden  piles  in  ranks  on  the  slopes  to  break  the 
force  of  the  slides.  Avalanche  snows  often  outlast  the  sum- 
mer at  the  base  of  cliffs. 

18.  (6)  Electrical  discharges. — These  are,  so  far  as  pres- 
ent investigations  have  gone,  a  minor  agent  in  weather- 
ing, although  they  are  of  considerable  interest.  If  the 
discharge  has  passed  through  sand  or  through  consolidated 
rocks  in  exposed  situations,  small  branching  tubes  lined 
with  glassy  material  may  be  found  descending  into  the 


WEATHERING 


27 


earth.  The  glassy  lining  is  caused  by  the  melting  of  the 
minerals  composing  the  rock  or  sand  in  the  path  of  the 
current.  Such  tubes  are  called  fulgurites,  and  have  been 
found  on  the  summit  of  Little  Ararat  and  in  the  Cascade 
Range  of  Western  America.  An  irregular  train  of  fulgu- 
rites has  been  found  leading  off  from  a  tree  which  was 


FIG.  13.— St.  Peter  sandstone,  Iowa.     Effects  of  joints  and  bedding  planes  on 
weathering.     From  Iowa  Geological  Survey. 

struck  by  lightning  in  Florida.  Sometimes  glassy  patches 
or  beads  are  formed  instead  of  tubes.  Electrical  shocks 
may  extensively  shiver  rock  masses  among  high  mountains. 


2g  GEOLOGY 

19.  Favorable  conditions  for  weathering.— It  is  evident 
that  the  weathering  agents  will  vary  in  their  efficiency  and 
grouping.  It  is  important  for  the  student  to  recognize  the 
interplay  of  the  several  forces  and  their  widespread  action. 
Rocks  crumble  much  more  readily  because  of  the  bedding 
and  joint  planes  which  intersect  them.  A  perfectly  solid 
mass  of  rock  would  be  attacked  with  difficulty  except  at 
the  surface.  As  it  is,  the  thickest  formations  and  greatest 
mountains  yield  to  the  silent  industry  of  Nature's  tools. 
Water  and  air  surely  find  the  smallest  openings  in  the 
rock,  and  there  can  be  but  one  result.  Geologists  are 
not  agreed  as  to  whether  rocks  weather  more  readily  in 
warm  or  cold  climates.  It  may  be  safe  to  say  that  chem- 
ical decay  proceeds  more  rapidly  in  warm  regions,  where 
vegetation  decays  rapidly  and  solution  is  active ;  while 
mechanical  disintegration  is  more  favored  in  temperate 
and  arctic  climates,  with  their  powerful  frosts  and  large 
variations  of  temperature. 


EFFECTS  OF  WEATHERING 

20.  (1)  Color  of  rocks. — The  color  of  rock  masses  as- 
sumes almost  as  large  variety  as  is  found  in  the  vegetable 
world.  Brown,  blue,  green,  gray,  yellow,  and  red,  in  all 
shades  and  combinations,  come  before  the  eye  of  the  geolo- 
gist until  they  baffle  description.  Pure  black  and  white 
rocks  are  not  very  uncommon.  Tones  are  usually  neutral 
and  harmonious,  though  brilliant  effects  are  sometimes  seen, 
as  among  the  Colorado  plateaus. 

Color  is  due  in  part  to  the  minerals  originally  compos- 
ing the  rock,  and  in  part  to  later  changes.  Some  of  these 
changes  fall  under  the  head  of  weathering.  Thus  the  clay 
at  the  top  of  a  pit  may  be  yellow  or  brown,  while  at  the 
bottom  it  is  blue,  because  there  protected  from  water,  air, 
frosts,  and  the  penetration  of  roots.  According  to  Merrill, 
the  Berea  sandstones  of  Ohio  are  gray  or  bluish  gray  below 


WEATHERING 


29 


the  drainage  level  of  the  quarries,  and  buff  above.  In  a 
great  number  of  cases  the  color  of  rocks  is  dependent  upon 
changes  produced  by  the  exposure  of  the  compounds  of 
iron  which  they  contain.  Some  soils  of  nonglacial  regions 
have  a  prevailing  red  color.  Some  sands  and  gravels  lying 


FIG.  14.— Vertical  strata  near  Manitou,  Col.,  showing  down-hill  creep  and  the 
weathering  of  hard  and  soft  layers.    Photograph  by  the  author. 

beneath  peat  beds  have  been  perfectly  bleached,  the  iron  in 
them  being  made  soluble  by  the  vegetable  acids  descending 
from  the  peat. 

21.  (2)  Soils. — Weathering  is  largely  instrumental  in 
the  formation  and  exhaustion  of  soils.  The  term  soil  is 
often  loosely  applied  to  the  whole  sheet  of  crumbled  rock 
material  which  mantles  most  of  the  rocky  foundation  of  the 
lands.  The  thickness  of  this  mantle  varies  from  zero  to 
several  hundred  feet.  It  is  produced  by  the  disintegration 


30  GEOLOGY 

and  decay  of  rocks,  and  is  largely  due  to  the  forces  whose 
total  effect  we  have  called  weathering. 

But  the  term  soil  is  better  restricted  to  the  thin  layer 
of  fine,  dark  material  which  chiefly  supports  plant  life.  It 
is  finer  because  more  thoroughly  weathered  and  broken 
down.  It  is  dark  in  color  chiefly  because  of  the  decaying 
vegetable  matter  which  it  contains,  and  without  which  it 
could  not  be  a  true  and  productive  soil.  Soils  may  be 
largely  derived  from  the  decay  of  the  underlying  rocks,  or 
they  may  be  partly  composed  of  matter  brought  from  a  dis- 
tance. Pebbles  and  bits  of  mineral  and  rock  constantly 
break  up  and  furnish  the  soils  with  fine,  soluble  material 
which  the  rootlets  of  plants  can  use.  Weathering  is  thus 
directly  essential  to  the  support  of  living  creatures  on  the 
earth.  On  the  other  hand,  some  of  the  weathering  processes 
may  be  destructive  of  soils.  Thus  by  solution  the  soils  of  a 
limestone  country  may  at  last  require  the  artificial  applica- 
tion of  lime.  Small  parts  of  the  soil-cap  are  constantly 
removed,  especially  on  sloping  fields,  and  the  soils  would 
in  the  end  be  destroyed  but  for  the  renewal  which  takes 
place  through  weathering. 

22.  (3)  Appearance  and  durability  of  building  stones. — 
When  stones  are  removed  from  their  protected  situation 
in  the  quarry  and  put  into  a  wall,  destruction  begins.  In 
the  end  no  rocks  can  withstand  it,  though  some  are  rela- 
tively permanent.  The  choice  of  building  stones,  there- 
fore, is  the  choice  of  the  better  appearing  and  less  destructi- 
ble. But  in  all  cases  a  few  years  will  bring  traces  of  decay. 
Obscure  bedding  planes  and  joints  come  to  light,  flakes  peel 
off,  changes  of  color  ensue,  surfaces  grow  dull,  and  after  a 
time  some  blocks  crumble  and  restoration  is  necessary. 

Let  the  student  discard  the  notion  that  any  rocks  are 
permanent.  Let  him  observe  the  walls  and  buttresses  of  a 
stone  church,  or  the  approaches  and  fronts  of  a  city  block, 
or  the  monuments  in  a  cemetery,  and  he  will  find  many 
illustrations.  The  firmest  granite  is  an  aggregate  of  scv- 


WEATHERING 


31 


eral  minerals,  which  in  time  will  break  apart.  The  finest 
marbles  are  limestone,  and  therefore  soluble.  Sandstones 
are  but  clinging  sands,  and  must  crumble  when  the  binding 
cement  dissolves.  Porous  rocks  which  take  up  water  are 
soon  destroyed  by  changes  of  temperature.  The  obelisk, 


FIG.  15.— Weathered  remnants  of  inclined  beds,  Garden  of  the  Gods,  Col. 

which  was  removed  from  the  dry  air  and  even  temperature 
of  Egypt,  would  soon  crumble  in  Central  Park  without 
artificial  protection.  In  Oxford,  England,  some  college 
walls  of  the  last  two  centuries  have  crumbled  seriously, 
while  portions  of  the  cathedral  and  of  some  ancient  defen- 
sive towers  have  stood  for  many  centuries.  Here  the  dif- 


32  GEOLOGY 

ference  is  in  the  material.  Buildings  may  be  everywhere 
seen  in  whose  walls  the  blocks  stand  on  edge  and  rapidly 
crumble,  whereas  if  laid  upon  their  natural  beds,  as  in  the 
quarry,  they  would  endure  indefinitely.  Climate,  choice 
of  material,  and  manner  of  laying  are  therefore  important 
considerations  in  building  with  stone. 

23.  (4)  Denudation. — The  removal  of  material  and  con- 
sequent lowering  of  a  land  surface  is  called  denudation. 
As  running  water  is  the  chief  means  of  transport,  the  sub- 
ject will  be  more  fully  treated  under  that  head.  But  it  is 
important  here  to  observe  that  weathering  is  chiefly  re- 
sponsible for  breaking  up  the  rocky  matter,  rendering  it 
accessible  to  the  transporting  agent.  The  earthy  debris 
found  everywhere  over  the  rocky  crust  of  the  globe  shows 
that  the  agents  of  removal  can  not  keep  pace  with  the  pro- 
cess of  destruction. 

Disintegration  can  not  proceed  without  limit  if  the 
material  be  not  at  the  same  time  removed,  because  there  is 
a  limit  of  depth  at  which  weathering  agents  are  effective. 
But  decomposition  has  gone  on  at  considerable  depths,  as 
many  observations  have  shown.  We  take  the  following 
from  Merrill :  "  In  the  work  of  grading  the  streets  in  the 
extensions  of  the  city  of  Washington,  masses  of  strongly 
foliated  granites,  so  soft  as  to  be  readily  removed  with  pick 
and  shovel,  would  be  cut  through,  which  yet  showed  every 
vein  or  other  structural  detail  as  plainly  marked  as  in  the 
original  rock,  and  it  was  only  by  thrusting  one's  cane  or 
other  implement  into  it  that  its  thoroughly  decomposed 
condition  became  apparent." 

Shale  rock  in  Brazil  is  reported  as  turned  into  clay  to  a 
depth  of  394  feet.  According  to  Geikie,  the  kaolin  arising 
from  the  decay  of  granite  is  sometimes  found  to  a  depth  of 
600  feet.  These  latter  are  somewhat  exceptional  cases. 
We  must,  however,  remember  that  where  removal  occurs 
promptly  strata  many  thousand  feet  thick  may  in  succes- 
sion disintegrate  and  be  carried  away. 


34  GEOLOGY 

In  the  study  of  weathering,  mere  disintegration  by  me- 
chanical means,  such  as  expansion  by  frost,  should  be  dis- 
tinguished from  decay  of  a  chemical  sort,  as  by  oxidation, 
although  in  Nature  both  processes  go  on  together. 

24.  (5)  Topography. — The  evolution  of  land  forms  under 
the  operation  of  all  the  geological  forces  will  be  treated  in 
a  subsequent  chapter.  But  we  must  now  call  attention  to 
the  important  effects  which  the  varying  resistance  of  the 
rocks  produce  upon  the  landscape.  East  of  the  Catskill 
Mountains  the  Hudson  Valley  is  many  miles  in  width.  In 
the  Highlands  the  valley  sides  rise  abruptly  from  the  edge 
of  the  water.  This  strong  contrast  in  scenery  is  due  to  the 
ready  yielding  of  the  rocks  to  weathering  in  the  Catskill 
region  and  the  stubborn  resistance  of  the  tough  rocks  of 
the  Highlands.  Central  Pennsylvania  is  a  region  of  deep 
valleys  alternating  with  mountain  ridges.  The  rocks  of 
the  region  have  been  more  or  less  steeply  turned  on  edge, 
and  the  soft  beds  have  weathered  and  crumbled  away, 
leaving  the  hard  beds  in  high  relief.  The  streams  have 
had  much  to  do  with  erosion  here,  and  are  wholly  the 
means  of  carrying  away  the  land  waste,  but  the  topography 
depends  primarily  on  weathering.  In  the  dry  regions  of 
the  West  vast  piles  of  horizontal  strata  have  been  carved 
into  remnants,  often  of  fantastic  form,  making  what  is 
known  as  the  "  Bad  Lands."  Without  a  protecting  mantle 
of  vegetation,  exposed  to  sun  and  frost,  with  occasional 
storms  and  spasmodic  torrents  for  the  transportation  of 
debris,  we  have  here  also  a  conspicuous  example  of  weath- 
ering. 

Sheets  and  dikes  of  ancient  lava  are  often  either  harder 
or  softer  than  the  rocks  with  which  they  are  associated. 
They  thus  may  cap  "  table  mountains,"  so  called,  or  form 
outstanding  walls  or  sunken  trenches,  according  to  their 
relative  degree  of  hardness.  North  and  South  Table  Moun- 
tains, at  Golden,  Colorado,  illustrate  the  former  case,  and 
some  lakes  and  river  channels  of  the  Hudson  Bay  region 


36 


GEOLOGY 


are  examples  of  the  latter.  The  remnants  of  horizontal 
beds  above  mentioned  may  dwindle  until  they  become  very 
narrow  and  frail,  as  may  be  seen  in  the  dells  of  the  Wis- 
consin River  and  many  parts  of  the  Rocky  Mountain  region. 
Those  pillars  at  length  fall,  and  the  stones  composing 
them  lose 'their  corners  by  weathering,  and  we  thus  have 
erosion  bowlders,  similar  in  form  to  some  that  have  been 
rounded  by  waves  and  glaciers.  A  bed  of  limestone  in  a 
field  unprotected  by  soil  may  weather  along  intersecting 
systems  of  joints  until  all  the  blocks  are  well  rounded, 
giving  us  a  field  of  bowlders  which  have  been  formed  in 
place. 


FIG.  19.— Erosion  pillar  with  resistant  cap. 
Conglomerate  of  Monument  Park,  Col. 


CHAPTER  III 

RIVERS 

25.  Definition. — By  rivers  is  here  meant  all  waters  flow- 
ing on  the   surface   of  the  lands.     In  this  sense  a  river 
includes  not  only  the  trunk  stream,  but  all  branches  and 
streamlets  from  its  sources  to  its  issue  in  the  ocean  or  inland 
sea.     If  several  rivers  are  said  to  flow  into  another,  as  the 
Ohio  and  others  into  the  Mississippi,  this  is  a  distinction 
for  convenience.     What  we  really  have  is  one  compound 
stream  or  river  system.     Strictly,  also,  many  lakes  are  only 
temporary  expansions  of  rivers,  and  hence  all  rivers  truly 
terminate  at  the  ocean  border.     As  commonly  used,  a  river 
denotes  any   considerable   stream,  and  yet  relatively,  for 
many  rivers  of  small  countries  would  be  called  creeks  or 
brooks  elsewhere.     Thus  most  rivers  of  Great  Britain  are 
small,  and  are  important  only  because  they  are  tidal  near 
their  mouths,  and  become  thus  the  servants  of  commerce. 

26.  Matter  and  energy. — The  chief  principle  of  science 
is  that  matter  and  energy  may  suffer  changes  of  form,  but 
are  in  themselves  indestructible.     Thus  water  may  be  found 
as  vapor,  as  liquid,  or  as  solid,  having  in  each  condition  its 
appropriate  geological  effects,  but  no  atom  is  ever  lost.     So 
also  force,  seen  now  as  heat,  may  reappear  as  light,  elec- 
tricity, motion,  or  chemical  reaction.     By  the  sun's  heat 
water  is  evaporated,  chiefly  from  the  ocean,  in  less  degree 
from   the   lands  ;   the  vapor   is   borne   over   land  and  sea 
by  the  winds,  is  condensed  by  cooling,  and  falls  as  rain, 
snow,   or  hail.      The  gathering  of  the  raindrops  on   the 

4  37 


g8  GEOLOGY 

earth's  surface  gives  origin  to  rivers.  The  rivers  run  to 
the  sea,  and  on  their  course  accomplish  the  most  important 
work  which  the  geologist  has  to  study.  It  is  well  here  to 
observe  that  the  force  applied  appears  first  as  heat  expended 
to  raise  the  water,  and  reappears  in  the  fall  of  rain  and  flow 
of  streams.  We  speak  of  the  geological  work  of  rivers,  but 
we  really  deal  with  force  in  the  forms  of  heat  and  gravita- 
tion, of  which  the  watery  matter  is  only  a  medium  of  appli- 
cation. 

27.  History  of  a  drop  of  rain  water.— If  water  lifted  from 
the  sea  falls  directly  back,  it  has  no  geological  function 
save  as  a  part  of  the  sea,  whose  work  we  shall  later  study.    If 
carried  over  the  land,  it  may  be  re-evaporated  in  the  course 
of  its  fall,  or  it  may  reach  the  surface  of  the  earth.     In  the 
latter  case  the  geological  activity  begins.     It  may  soak  into 
the  ground,  and,  after  a  passage  more  or  less  long  and  deep, 
emerge  in  a  spring  and  become  part  of  a  river ;  or  it  may 
be  taken  into  the  tissue  of  a  plant  and  be  re-evaporated 
into  the  air ;  or  it  may  run  directly  off,  helping  to  form  a 
rill  and  then  a  river.     It  is  the  last  case  with  which  we  are 
now  concerned,    \yhen  the  drop  of  water  has  passed  from 
the  sea  to  the  clouds,  has  traveled  over  the  lands,  fallen  to 
the  earth,  and  returned  by  a  river  to  the  sea,  it  may  be  said 
to  have  passed  thr6ugh  a  cycle  of  activity. 

28.  Rainfall  and  run-off. — The  amount  of  rain  that  falls 
at  a  given  point  depends  upon  its  nearness  to  the  sea,  upon 
the  latitude,  altitude,  temperature,  surrounding  topography, 
and  other  conditions.     In  the  eastern  United  States  the 
annual  rainfall  is  from  40  to  50  inches.     In  much  of  the 
West  it  is  about  20  inches  or  less.     In  England  and  Wales 
the  amount  is  about  32  inches.     Along  the  north  Pacific 
coast  of  America,  on  some  South  American  borders,  and  in 
parts  of  India  it  is  more  than  100  inches.     In  the  latter 
region  at  some  points  more  than  30  feet  per  year  fall. 
Much  of  the  rainfall  soaks  away,  or  is  at  once  evaporated. 
The  "run-off"  only  is  available  for  stream  work,  and  the 


RIVERS  39 

percentage  is  variable,  depending  on  climate  and  soil. 
According  to  Dana,  one  third  to  two  fifths  of  the  rainfall 
runs  off  in  the  temperate  latitudes.  One  fourth  is  the  pro- 
portion for  the  Mississippi  basin,  and  one  third  for  that  of 
the  Seine.  In  very  dry  regions  the  percentage  is  much  less. 

29.  Parts  of  a  river. — A  large  river  which  drains  areas 
of  continental  extent  shows  great  variety  in  its  different 
parts.     We  may  call  such  a  stream  a  typical  river.     The 
Amazon,  the   Rhine,  and  the   Ganges  are  good  illustra- 
tions.    They  rise  in  high   mountains,  flow   swiftly  down 
narrow  gorges,  cross  the  lower  lands  by  spacious  valleys, 
and  swing   easily  over  broad  alluvial   plains   to  the  sea. 
Geikie  has  called  these  parts  the  mountain  track,  the  val- 
ley track,  and  the  plain  track  of  a  river.     They  show  infi- 
nite variety  in  form  and  relative  length,  and  one  or  more 
of  them  may  be  absent.     Thus  the  Susquehanna  and  the 
Eed  River  of  the  North  do  not  rise  in  mountains,  though 
there  are  precipitous  ravines  at  some  sources  of  the  former. 
The  western  Andes  also  are  drained  by  torrential  streams 
which  pass  almost  immediately  to  the  sea.     In  the  median 
or  valley  track  of  a  single  river  great  difference  may  appear, 
dependent  on  rock  structure.     Thus  the  Yukon  is  a  case 
in  point.     The  valley  track  is  many  hundred  miles  in  length, 
"bordered  in  places  by  magnificent  bluffs  of  hard  rock, 
which  intervene  between  long  reaches  where  the  valley  is 
several  miles  broad  and  has  been  excavated  in  softer  beds." 
Similarly,  quiet  reaches  of  a  river  alternate  with  rapids  and 
waterfalls,  and  sections  of  a  river  are  by  various  means 
transformed  into  lakes. 

30.  Vigor  and  continuity  of  river  action. — The  best  evi- 
dence of  this  is  in  the  facts  which  follow.     A  river  is  the 
most  active,  varied,  and  we  may  almost  say  vital  thing  in 
the  realm  of  inorganic  nature.     In  the  making  up  of  new 
lands,  the  remodeling  of  the  old,  and  in  its  control  over 
human  life,  it  is  pre-eminent  among  geological  facts  and 
forces. 


FIG.  20.-Torrent  courses  and  waste  slopes,  with  avalanche  snows,  Guttannen,  Switzerland. 


RIVERS  41 

EROSION  BY  STREAMS 

31.  Definition  and  examples. — The  most  general  term  for 
the  depression  through  which  a  stream  runs  is  valley.     A 
valley  may  be  deep  or  shallow,  wide  or  narrow,  and  may 
have  steep  or  flaring  sides  of  various  form.     A  narrow,  deep 
valley  may  be  called  a  gorge  (gullet),  gulch,  ravine,  or 
canon.    The  term  ravine  refers  to  the  violent,  tearing  action 
by  which  such  a  valley  was  made.     The  term  canon  is  of 
Spanish  origin,  and  is  most  commonly  used  of  the  deep 
gorges  of  the  West.     That  most  valleys  are  due  to  the 
rivers  which  are  flowing  in  them  was  not  appreciated  until 
the  earlier  part  of  the  nineteenth  century.     Now  it  seems 
axiomatic  that  the  Connecticut,  Susquehanna,  and  Ohio 
valleys,  and  the  gorges  of  Trenton,  Niagara,  and  the  Colo- 
rado, are  mainly  due  to  stream  erosion.     We  shall  now  in- 
quire how  such  work  is  done. 

32.  Means  of  river  erosion. — Water  which  carries  no  sus- 
pended rocky  matter  may  erode  a  considerable  channel  in 
soils,  sands,  or  gravels,  especially  if  the  current  be  swift. 
It  would  thus,  however,  pick  up  tools  which  would  enable 
it  effectively  to  cut  into  the  harder  rocks.     Every  particle  of 
rock  serves  as  a  rasp  when  driven  by  the  current  against 
the  rocks  at  the  bed  or  border  of  the  stream.     The  friction 
of  contact  wears  away  small  fragments,  and  by  long  con- 
tinuance   the   action  accomplishes  much   deepening  and 
widening  of  the  channel.     The  water  of  Niagara  Eiver  has 
lost  its  load  of  rocky  debris  in  the  quiet  waters  of  Lake 
Erie,  and  hence  has  scarcely  sunk  its  channel  below  the 
surface  even  by  the  raging  torrent  of  the  rapids  above  the 
fall.     The  great  erosion  accomplished  below  the  falls  is  due 
to  special  conditions  which  will  hereafter  be  explained.     In 
like  manner  the  St.  Lawrence,  receiving  the  clear  waters  of 
Lake  -Ontario,  accomplishes  little  erosion  at  the  Thousand 
Islands.     The  Connecticut,  on  the  other  hand,  loaded  with 
waste  from  the  mountains  of  New  England,  has  excavated 


42  GEOLOGY 

a  great  valley.  Sandy  matter  wielded  as  an  erosion  tool  by 
flowing  water  is  analogous  to  similar  material  driven  by  a 
stream  of  air  in  the  natural  sandblast. 

River  erosion  is  also  favored  by  the  presence  of  joints 
and  bedding  planes.  These  planes  are  opened  by  solution 
and  by  mechanical  wear  as  above  described,  so  that  it  is  no 
uncommon  thing  to  see  the  bottom  of  a  stream  paved  with 
rectangular  blocks  more  or  less  isolated  from  each  other. 
Water  in  portions  of  shallow  streams  is  often  frozen  to  the 
bottom  in  winter.  The  ice  attaches  itself  about  the  blocks 
of  rock  and  in  the  floods  of  the  spring  buoys  them  up,  or 
at  least  aids  the  swift  current  in  dislodging  them.  They 
are  then  tumbled  against  one  another,  subjected  on  every 
side  to  filing  and  solution,  to  alternations  of  moisture  and 
dryness,  heat  and  cold.  Thus  weathering  co-operates  with 
the  direct  work  of  streams.  Particularly  do  the  solvent 
processes  at  all  times  accompany  the  more  obvious  mechan- 
ical disintegration  which  takes  place.  That  events  do  not 
proceed  from  a  single  cause,  but  are  rather  the  resultant  of 
many  forces,  is  a  cardinal  principle  in  geology.  For  con- 
venience we  treat  one  process  at  a  time,  but  we  must 
remember  that  other  processes  are  always  at  work. 

33.  Overloaded  streams.— As  we  shall  see,  in  studying  the 
transportation  of  rivers,  streams  sometimes  carry  a  great 
burden  of  earthy  matter.     The  waters  are  loaded  to  their 
full  capacity,  and  no  energy  is  left  for  erosive  work.     A 
medium  amount  of  transported  material  therefore  gives  to 
flowing  water  its  highest  abrasive  power. 

34.  Mutual  abrasion  of  particles.— Transported  fragments 
wear  off  particles  from  the  rocky  bed  of  streams,  and  in  turn 
suffer  loss  themselves,  not  only  by  friction  against  the  bot- 
tom but  by  being  rubbed  upon  each  other.    Vast  quantities 
of  rock  flour  or  fine  mud  are  thus  made  and  strewn  along 
the  valley  and  eventually  carried  out  to  sea.     Experiment 
has  shown  that  when  pieces  of  granite  and  quartz  were  rolled 
over  each  other  in  water  through  a  distance  of  15$  miles,  the 


FIG.  21. — Narrow  gorge  in  horizontal  clruta.  Ausable  Chasm,  N.  Y. 

(Copyright  by  S.  R.  STODDAED,  Glen.  Falls.  N.  Y.) 


44 


GEOLOGY 


fragments  of  granite  lost  four  tenths  of  their  weight  and 
became  rounded  like  river  pebbles.  The  water  was  filled 
with  a  very  fine  mud,  which  remained  suspended  for  sev- 
eral days.  Cubes  of  rock  placed  with  water  in  a  revolving 
receptacle  will  become  almost  perfect  spheres,  the  "  mar- 
bles "  of  children's  play.  The  banks  and  shoals  of  swift 
streams  will  supply  abundant  illustrations  in  pebbles  of 
remarkably  symmetrical  forms,  which  can  sometimes  be 
traced  to  their  sources  up  the  stream,  and  the  distance  of 
transport  can  be  thus  determined.  At  points  along  the 
Khine,  according  to  Geikie,  the  grinding  of  the  pebbles 
upon  each  other  can  be  heard  by  an  observer  who  holds  the 
ear  to  the  bottom  of  an  open  boat. 

35.  Down-cutting  by  streams.— We  are  now  ready  to  see 
how  a  river  deepens  its  channel  and  is  ever  sinking  its  bed 
toward  the  sea  level.  The  higher  the  river  bed  above  the 
sea,  the  greater  is  the  average  velocity  of  the  stream  and 
the  force  with  which  it  applies  its  erosive  tools.  Hence 
valleys  are  commonly  deepening  most  rapidly  in  mountains 
and  near  the  head  waters  of  streams.  Perpetual  filing  of 
surfaces  and  upturning  of  slabs  and  blocks  of  rock  bring 
the  sure  result.  Under  certain  conditions  slush  and 
ground  ice  or  anchor  ice  form  at  the  bottoms  of  streams  and 
aid  in  floating  stones  which  are  more  or  less  inclosed  by  it. 
Valleys  are  deepened  in  an  important  way  by  the  recession 
of  waterfalls.  Along  swift  portions  of  a  stream  the  forma- 
tion of  potholes  aids  in  sinking  the  channel.  At  such  points 
sharp  eddies  form  with  a  downward  spiral  movement  of  the 
waters.  The  eroding  implements  are  wielded  with  especial 
effect  in  such  situations,  and  a  depression  is  readily  hollowed 
out  in  the  bedrock  beneath.  As  this  deepens  it  catches  and 
holds  stones  of  considerable  size,  which  maintain  their  gy- 
rating movements  even  after  the  pit  becomes  deep  in  propor- 
tion to  its  width.  The  work  often  goes  on  vigorously  in  the 
spring,  while  in  summer  and  autumn  we  find  pools  of  still 
water,  at  the  bottom  of  which  the  rolled  stones  may  be  seen, 


RIVERS  45 

These  are  continually  worn  out,  and  others  from  up  the 
stream  take  their  places.  The  pits  may  attain  a  depth  of 
20  to  30  feet,  and  the  diameter  will  vary  from  a  few  inches 
to  many  feet.  Potholes  of  10  feet  in  depth  may  not  have  a 
maximum  diameter  of  more  than  2  feet.  If  layers  of  vari- 
able hardness  are  encountered  the  diameter  may  show  abrupt 
variations.  At  Little  Falls,  X.  Y.,  where  the  waters  of  the 
Mohawk  descend  over  hard  rocks,  there  is  a  noteworthy 
display  of  potholes.  Some  are  20  or  30  feet  in  diameter, 
irregular  in  form,  as  if  several  adjacent  pits  had  been 
merged  into  one  by  the  wearing  out  of  their  bounding 
walls.  Another  of  20  feet  in  depth  opens  at  a  height  of  60 
feet  above  the  river,  proving  in  an  interesting  manner  the 
former  presence  of  the  river  at  that  altitude.  Similarly  a 
curve  of  large  radius  is  often  hewn  out  of  a  cliff  at  the 
base  of  a  waterfall. 

36.  The  widening  of  valleys.— By  downward  erosion  along 
a  belt  covered  by  the  stream  at  a  given  time,  only  a  nar- 
row gorge  could  result.  But  other  factors  enter  in  to  give 
a  valley  width.  Here  we  have  to  do  partly  with  weather- 
ing and  partly  with  the  behavior  of  the  stream.  It  is  not 
quite  true  to  say  that  a  river  makes  a  wide  valley,  though 
it  is  the  chief  instrument.  Solution  on  the  borders  of  a 
stream,  the  offthrust  by  frosts  and  by  roots  of  trees,  the 
burrowing  of  animals  and  the  creep  of  soils,  have  greatly 
to  do  with  the  result  and  should  have  wider  recognition 
as  agents  in  valley  making.  Returning  now  to  the  direct 
action  of  the  stream,  we  observe  that  the  river  tools  not 
only  file  the  bottom,  but  the  banks,  and  thus  widen  the 
channel.  Wherever  ice  forms  over  rivers  it  carries  off 
attached  rock  fragments  from  its  borders,  and  exerts  a 
stupendous  grinding  and  plucking  force  as  it  breaks  up. 
A  similar  but  quiet  thrust  is  exerted  when  the  ice  shrinks 
and  cracks  in  time  of  intense  cold,  and  water  fills  the 
cracks  and  expands  as  it  freezes,  the  whole  mass  being  thus 
thrust  outward  against  the  banks. 


45  GEOLOGY 

We  have  now  to  notice  the  most  effective  way  in  which 
a  river  widens  its  valley.  It  is  constantly  grazing  with 
greater  force  on  one  or  other  of  its  banks.  This  is  true  in 
all  valleys,  but  more  obviously  in  those  which  have  consid- 
erable width,  where  the  stream  swings  to  and  fro,  cutting 
a  curved  slice  now  out  of  one  bank  and  now  out  of  the 
other.  The  location  of  the  curves  shifts  from  time  to  time, 
and  thus  in  the  end  the  sides  of  the  valley  are  encroached 
upon  at  all  points.  Steep  bluffs,  describing  a  curve,  may 
often  be  seen  at  some  distance  from  the  stream.  A  swamp 
or  lagoon  below  the  bluff  marks  the  recent  presence  there 
of  a  river,  which  is  now  at  work  at  another  point.  In  time 
such  bluffs  are  softened  by  weathering,  and  become  a  part 
of  the  flaring  walls  of  an  old  valley. 

37.  Limits  of  vertical  and  lateral  erosion. — Whenever  a 
river  or  a  part  of  it  attains  the  sea  level  it  ceases  to  sink 
its  channel,  and  is  said  to  have  reached  its  base  level  of 
erosion.     Base  level  is  an  important  term  and  means  the 
level  below  which  subaerial  erosion   can  not  take  place. 
This  limit  is  practically  reached  when  sea  level  is  approxi- 
mated and  the  stream  becomes  sluggish.     The  process  of 
widening  will,  however,  continue.     Thus  the  Connecticut 
Eiver  is  close  to  base  level  throughout  its  course  across 
Massachusetts  and  Connecticut,  but  is  yet  actively  widen- 
ing its  valley  in  those  States,  while  farther  north  vertical  as 
well  as  lateral  erosion  is  vigorously  carried  on.     If  a  stream 
crosses  a  resistant  barrier,  its  bed  for  some  distance  above  will 
in  time  wear  down  to  this  level,  which  may  be  called  a  local 
base  level.    Thus  a  river  may  descend  to  the  sea  by  several 
sections  resembling  steps,  in  each  of  which  deepening  is  slow 
and  widening  is  active.     In  time,  however,  all  barriers  will 
be  cut  away,  and  the  ultimate  base  level  be  approached. 
Lateral  erosion  of  a  valley  will  continue  so  long  as  high 
ground  remains  between  it  and  its  neighboring  valley. 

38.  River  erosion  not  continuous.— This  is  true  whether  we 
consider  time  or  space.     Streams  often  erode  actively  only 


RIVERS  47 

in  time  of  flood.  Nearly  all  the  destructive  work  of  a  year 
may  be  done  in  a  few  days  at  the  end  of  winter,  or  a  few 
periods  of  heavy  rain.  Stream  beds  which  are  dry  in  Au- 
gust may  be  filled  by  a  destructive  torrent  in  April.  Minia- 


L 


tard  growth  of  gorge  whose  lower  slopes  are  wooded, 
Palenville  Clove,  Catskill  Mountains. 


ture  caftons  may  be  formed  on  a  hillside  during  a  single 
storm,  crops  are  uprooted,  and  bridges  and  culverts  are 
swept  away.  Disastrous  results  often  follow  a  break  in  the 
banks  of  a  canal.  Repetitions  of  such  effects  during  long 
periods  in  Nature  are  ample  for  making  the  gorges  of  the 


48 


GEOLOGY 


Hudson,  the  Yellowstone,  the   Columbia,  and  the   Colo- 
rado. 

Xor  does  erosion  go  on  throughout  the  extent  of  a  river. 
It  is  active  in  the  torrent  section,  intermittent  in  the  val- 
ley section,  and  nearly  zero  as  the  river  nears  the  sea.  In 
the  middle '  portions  of  the  river  deposit  may  go  on  at  one 
season  and  erosion  at  another.  Wherever  the  stream  bed  is 
composed  of  mud,  gravel,  and  stones,  we  may  have  removal 
or  deposit  of  these  materials,  but  no  cutting  of  the  bed 
rock.  If  the  under  rock  is  exposed,  we  may  know  that  solu- 
tion is  active,  and  that  more  or  less  mechanical  erosion  is 
going  on. 

TRANSPORTATION  BY  STREAMS 

39.  Streams  are  the  common  carriers  of  the  continents. 
According  to  their  velocity  and  volume  and  the  supply  of 
material,  they  ever  go  loaded  toward  the  sea,  transferring 
the  material  of  the  lands  to  the  marginal  bottoms  of  the 
ocean.     This  process  has  long  been  recognized  by  man. 

"  The  waters  wear  the  stones ; 
The  overflowings  thereof  wash  away 
The  dust  of  the  earth."— Job  xiv,  19. 

"  The  sound  of  streams  that  swift  or  slow 
Draw  down  vEonian  hills,  and  sow 
The  dust  of  continents  to  be." — Tennyson,  In  Memoriam. 

40.  Derivation  of  load.— The  rocky  materials  are  made 
ready  for  transportation  in  a  variety  of  ways,  partly  by  the 
destructive  work  of  the  stream  itself  as  already  described, 
partly  by  the  entire  assemblage  of  weathering   agencies, 
very  largely  in  some  ages  and  parts  of  the  world  by  gla- 
ciers, and  sometimes  by  volcanoes  and  earthquakes.     In 
some  cases  a  stream  is  more  than  equal  to  the  task  sup- 
plied, and  thoroughly  removes  the  loose  matter  from  its 
bed  and  border.     At  other  times  much  material  is  supplied, 
and  from  lack  of  volume  or  velocity  the  stream,  already 


RIVERS  49 

overloaded,  leaves  available  material  on  its  banks  untouched 
and  struggles  along  a  clogged  channel  toward  the  sea. 
The  Platte,  flowing  sluggishly  for  long  distances  through  a 
region  of  crumbling  rocks  and  soils,  well  illustrates  this 
principle.  Floating  ice  may  be  the  means  of  transport 
both  for  fine  materials  frozen  within  it  and  for  stones  much 
too  large  to  be  moved  by  a  stream  of  water  unaided.  A 
bowlder  on  the  banks  of  the  Yukon,  six  feet  in  diameter, 
was  thus  carried  scores  of  miles  from  its  parent  ledge. 
Swift  tributaries,  which  break  up  first  in  the  spring,  may 
sweep  much  debris  down  upon  the  still  frozen  surface  of 
the  main  river.  Avalanches  and  landslides  in  steep  moun- 
tain valleys  may  accomplish  similar  results.  Xo  illustra- 
tion of  the  carrying  power  of  water  is  more  valuable,  how- 
ever, than  the  wayside  rill  and  rising  meadow  brook,  clouded 
with  land  rubbish  after  every  heavy  rain.  Within  a  few 
feet  of  each  other  points  may  be  found  where  a  sluggish 
current  is  moving  only  fine  matter,  and  where  a  miniature 
torrent  is  pushing  forward  pebbles  of  considerable  size. 
Xo  observations  can  be  more  valuable  to  the  beginning 
student  than  such  simple  ones  which  are  possible  to  all. 

41.  Transporting  power. — This  depends  upon  velocity  and 
volume.  With  enlarging  rate  of  flow  the  size  of  fragments 
carried  increases  very  rapidly.  It  has  been  shown  that  the 
transporting  power  of  flowing  water  varies  as  the  sixth 
power  of  the  velocity.  Increase  of  volume  not  only  adds 
to  the  transporting  medium,  but  increases  velocity  by  de- 
creasing the  relative  amount  of  friction  between  the  stream 
and  its  bed.  According  to  Dana,  the  size  of  fragments  car- 
ried by  currents  of  given  velocities  is  as  follows  : 

Velocity  in  miles  Size  of  fragments, 

per  hour.  Diameter  in  inches. 

£ 0.016— fine  earth  or  clay. 

3. 0.064— fine  sand. 

2 0.6    —small  pebbles. 

4 2£      — large  pebbles. 


*IG.  23.-Torrent  bed,  ble  of  Skye,  showing  large  bowlders  moved  in 

time  .if  Hood. 


RIVERS  51 

We  have  also  to  remember  that  the  specific  gravity  of 
water  is  nearly  two  fifths  as  great  as  that  of  ordinary  rocky 
material.  Hence  a  stone  immersed  in  water  loses  a  large 
percentage  of  its  relative  weight,  and  tends  to  be  buoyed 
up  and  easily  borne  along.  Stones  are  often  rolled  over  and 
over  along  the  bottom,  or  borne  up  and  on  for  a  short  dis- 
tance, sinking  to  temporary  rest  again.  But  if  materials 
are  small  and  velocity  large  they  may  travel  long  distances 
without  coming  to  repose. 

42.  Amount  of  solids  discharged  by  a  river.— Several  classes 
of  material  must  here  be  taken  into  account:  (1)  Matter 
held  in  solution  ;  (2)  particles  held  in  suspension  ;  (3)  rock 
fragments  too  large  to  be  suspended  in  water,  but  rolled  or 
otherwise  propelled  along  the  bottom  ;  (4)  organic  matter 
— remains  of  plants  and  animals.  The  facility  with  which 
acidulated  waters  dissolve  carbonate  of  lime  and  other  min- 
erals has  been  considered  under  weathering.  Much  of  this 
dissolved  matter  finds  its  way  promptly  into  the  surface 
streams  and  is  carried  to  the  sea.  Russell  has  compiled 
the  following  table,  showing  how  much  matter  is  carried 
out  in  solution  annually  by  well-known  rivers  : 

Rhine 5,816,805  tons. 

Rhone 8,290,464  " 

Danube 22,521,434  " 

Thames 613,930  " 

Nile 16,950,000  " 

Croton  66,795  " 

Hudson 438,000  " 

Mississippi .  112,832,171  " 

Calcium  carbonate  leads  all  other  substances,  then  follow 
magnesium  carbonate,  organic  matter  and  silica,  with  small- 
er amounts  of  calcium  sulphate,  sodium  sulphate,  sodium 
nitrate,  potassium  sulphate,  sodium  chloride,  and  several 
others. 

The  amount  of  material  carried  in  suspension  varies 
much  according  to  season  and  the  rocks  of  the  region.  So 


52  GEOLOGY 

also  organic  matter  is  very  unequal  in  different  places, 
being  much  greater  amid  luxuriant  growths  of  warm  cli- 
mates. The  Red  River,  according  to  Dana,  was  in  1854 
obstructed  by  a  timber  raft  13  miles  long,  which  was  grow- 
ing at  its  upper  end,  and  breaking  up  and  sending  its  mate- 
rials to  the  Gulf  at  its  lower  end.  But  much  more  impor- 
tant than  such  isolated  cases  is  the  silent  and  constant 
carriage  of  twigs,  leaves,  grasses  and  roots,  bones  and  shells, 
as  well  as  decayed  and  dissolved  organic  matter.  This  is 
done  by  small  and  great  streams  everywhere. 

Computations  have  often  been  made  to  show  how  much 
matter  is  carried  in  all  these  ways  in  a  year  by  a  given 
river.  For  this  we  must  obviously  learn  the  amount  of 
water  discharged  and  the  percentage  of  solid  matter  in 
solution  and  suspension.  To  find  the  amount  of  water,  we 
must  determine  the  average  cross  section  of  the  stream  for 
a  year  near  its  mouth  and  the  average  velocity.  The  pro- 
duct of  the  two  gives  the  amount  of  water.  We  now  seek 
the  percentage  of  dissolved  matter  (by  analysis),  the  per- 
centage of  suspended  matter  (by  allowing  the  particles  to 
settle),  and  the  amount  pushed  on  the  bottom  (by  esti- 
mate). Adding  these  and  multiplying  into  the  total  dis- 
charge of  water  we  have  the  desired  result. 

Such  conclusions  only  roughly  approximate  the  truth, 
but  have  been  so  often  and  so  carefully  sought  that  they 
may  be  received  with  some  confidence.  One  of  the  most 
famous  calculations  of  that  sort  deals  with  the  Mississippi 
River,  and  was  made  many  years  ago  under  the  direction 
of  the  United  States  Government.  The  conclusion  was 
that  enough  solid  matter  is  yearly  transported  to  the  Gulf 
of  Mexico  to  make  a  column  1  mile  square  and  268  feet 
high,  without,  however,  taking  dissolved  matter  into  ac- 
count. 

Rate  of  denudation  of  land  surfaces.— With  unceasing  dis- 
integration and  transport  of  rocky  matter,  it  is  evident  that 
land  surfaces  will  be  lowered  toward  the  base  level  of  ero- 


RIVERS  53 

sion,  if  there  be  no  uplift  to  offset  the  work  of  destruction. 
The  time  thus  required  to  reduce  a  continent  to  sea  level 
is  a  point  of  great  geological  importance.  Interesting  re- 
sults have  been  reached,  though  it  is  not  possible  to  remove 
sources  of  error  and  doubt.  We  must  find  the  rate  of  removal 
as  above,  and  the  amount  of  matter  to  be  moved.  The 
factors  of  the  latter  are  the  area  and  average  height  of  the 
continent  or  particular  river  basin.  The  product  of  these 
gives  the  amount  of  matter  standing  in  relief  above  the  sea, 
and  this,  divided  by  the  amount  carried  in  one  year,  gives 
the  number  of  years  required  for  reduction  to  base  level  if 
the  rate  were  uniform.  But  we  must  remember  that  as 
the  process  goes  on  the  land  becomes  lower,  the  streams 
lose  velocity,  both  decay  and  transportation  are  retarded, 
and  the  rate  of  degradation  steadily  diminishes  to  the  end. 
We  may  arrive  at  the  result  in  a  slightly  different  man- 
ner, using  our  concrete  case,  the  Mississippi.  Taking  the 
column  of  matter  1  mile  square  and  268  feet  high,  we 
divide  the  height  by  the  number  of  square  miles  in  the 
basin  of  the  river  (including  all  its  branches),  and  find  that 
a  film  of  matter  j^7  of  a  foot  thick  is,  on  the  average, 
annually  pared  away  from  the  whole  area.  At  this  rate  a 
foot  would  be  taken  off  in  about  5,000  years.  If  dissolved 
matter  be  also  taken  into  account,  as  is  desirable,  Russell 
thinks  the  rate  would  be  increased  to  one  foot  in  4,000 
years.  We  can  see,  however,  that  in  some  distant  future 
the  plateaus  of  Kansas,  Nebraska,  and  the  Appalachian 
region  would  be  worn  down,  the  Kanawha  and  the  Tennes- 
see, the  Missouri  and  Arkansas  would  lose  much  of  their 
working  power,  and  a  foot  of  degradation  would  take  a 
much  longer  time.  This  may  be  shown  by  comparing  dif- 
ferent parts  of  the  United  States  as  regards  altitude  and 
vigor  of  river  action.  The  estimated  mean  height  of  cer- 
tain States  has  been  given  as  follows  : 


Colorado 6,800  feet. 

Oregon 8,300     " 

5 


New  York 900  feet. 

Florida 100    " 


54  GEOLOGY 

We  may  now  contrast  the  debris-covered  slopes  and  power- 
ful torrents  of  Colorado  and  Oregon  with  the  wooded  sum- 
mits and  more  moderate  streams  which  prevail  in  New 
York.  And  in  turn  we  may  set  the  gorges  of  the  Catskills 
and  the  Finger  Lake  region,  and  the  swift  waters  of  the 
upper  Hudson,  over  against  the  crawling  streams  and  slow 
degradation  of  Florida. 

With  the  Mississippi  we  should  compare  well-known 
rivers  in  other  lands.  The  rate  of  denudation  for  the  basin 
of  the  Po  is  estimated  at  one  foot  in  729  years.  The  Rhone 
is  believed  to  be  lowering  its  basin  at  the  rate  of  one  foot  in 
1,528  years,  and  the  Ganges  one  foot  in  1,880  years.  That 
stupendous  transfers  of  matter  from  land  to  sea  are  accom- 
plished by  prolonged  river  action  is  a  fundamental  principle 
of  geology. 

DEPOSITION  BY  STREAMS 

43.  The  law  of  deposition. — That  a  stream  drops  its 
load  in  proportion  as  it  loses  velocity  is  the  great  law  of 
deposition.  If  the  loss  be  slight,  only  the  coarsest  parts  of 
the  load  will  sink  to  rest.  If  the  loss  be  sudden  and  great, 
both  pebbles  and  fine  mud  will  be  laid  down  together. 
Compare  the  sediment  along  a  swift  part  of  a  rill  with  that 
which  settles  in  its  quiet  pools.  A  stream  may  lose  velocity 
by  change  of  declivity,  by  various  forms  of  obstruction,  by 
overloading  with  land  waste,  and  by  entering  a  body  of  still 
water.  From  the  nature  of  the  deposits  made,  we  may 
often  read  correctly  the  history  of  the  stream  that  made 
them.  The  presence  of  minerals  in  solution  in  water  re- 
sults in  a  variable  capacity  to  sustain  and  carry  rock  frag- 
ments. When  a  stream  enters  the  ocean  it  deposits  its 
load  both  because  of  loss  of  velocity  and  by  reason  of  the 
salt  in  the  marine  waters,  which  hastens  deposition.  This 
can  be  illustrated  by  filling  two  glass  jars,  one  with  fresh 
and  the  other  with  salt  water,  and  dropping  a  handful  of 
earth  into  each.  The  general  law  of  deposition  above  given 


RIVERS  55 

will  be  constantly  illustrated  as  we  study  the  various  forms 
of  deposit  by  streams.  In  this  connection  we  are  indebted 
to  Prof.  W.  M.  Davis  for  a  useful  phrase,  which  it  will  be 
well  for  the  student  to  retain  as  a  general  designation  of 
the  several  deposits  :  "  The  forms  assumed  by  the  waste  of 
the  land  on  its  way  to  the  sea."  Thus  all  unconsolidated 
surface  materials  may  be  regarded  truly  as  en  route  to  the 
ocean.  That  they  pause  in  a  field  or  river  bank  for  long 
periods,  or  that  they  may  move  with  extreme  slowness,  as 
in  the  creep  of  soils,  need  not  interfere  with  this  concep- 
tion, so  long  as  the  sea  is  the  sure  goal  of  land  waste. 
Geology  has  ample  stores  of  time  upon  which  to  draw  for 
all  her  processes. 

Soils  are  a  form  of  land  waste.  Their  origin  and  move- 
ments have  already  been  noticed,  and  we  may  pass  at  once 
to  those  sloping  banks  of  debris  known  as 

44.  The  talus. — The  word  means  literally  ankle,  and  is 
applied  in  geology  to  piles  of  rock  rubbish  which  have 
fallen  at  the  foot  of  cliffs.  The  talus  is  not  a  stream  de- 
posit like  those  which  follow,  but  it  is  so  closely  related  to 
streams  and  stream  work  that  it  is  convenient  to  study  it 
in  this  connection.  From  every  cliff  fragments  drop,  being 
loosened  in  ways  already  described.  If  there  be  a  powerful 
stream  at  the  base  it  may  carry  away  the  material  as  it  de- 
scends. Otherwise  it  accumulates  in  long,  sloping  banks 
under  the  cliff,  and  may  be  rough  and  stony  or  smooth  and 
more  or  less  covered  with  plants,  according  to  the  material. 
If  the  latter  be  coarse  and  angular  the  slope  will  be  steep. 
If  it  be  fine  and  rounded  or  dry  the  angle  with  the  horizon 
will  be  less.  The  angle  of  inclination  with  a  horizontal 
plane  which  loose  material  will  assume  is  important.  It  is 
often  called  the  angle  of  repose,  and  the  student  should 
accustom  himself  to  determining  it  by  measurement  and 
by  estimate. 

Inclinations  of  25°  to  35°  are  common.  If  talus  mate- 
rial is  supplied  rapidly  from  the  cliffs,  it  may  mantle  the 


GEOLOGY 


face  of  the  latter  almost  to  the  top.  The  stream  gathers 
its  load  from  the  lower  edge  of  an  adjacent  talus,  often 
taking  out  a  considerable  segment,  leaving  a  very  steep 


PIG.  24.— Cliff  and  tains,  Genesee  Gorge,  Rochester,  N.  Y. 
Contact  of  Medina  and  Clinton  strata. 

bank  next  the  stream.  A  noteworthy  talus  is  formed  by 
the  bowlders  of  trap  rock  which  fall  from  the  Palisades  of 
the  Hudson.  Photographs  of  Western  scenery  will  afford 
many  good  examples,  and  every  steep  bluff,  sharp  ravine,  or 
gravel  pit  will  give  illustrations  for  study. 

A  succession  of  cliffs  alternating  with  platforms,  as  on 
the  sides  of  some  canons  and  mountains,  will  give  a  suc- 
cession of  taluses.  As  the  material  works  down  by  frost 


RIVERS 


57 


and  "creep,"  it  may  descend  over  successive  cliffs  and 
taluses  until  it  reaches  the  drainage  stream.  As  the  sev- 
eral cliffs  yield  their  material  and  are  worn  away,  they  will 
be  more  and  more  obscured  by  waste,  the  several  talus 
slopes  will  merge,  and  we  shall  have  a  "graded  slope." 
Graded  slopes  may  also  arise  without  the  intervention  of 
successive  cliffs,  as  seen  in  the  rubbish  slopes  of  Pike's 
Peak  above  the  timber  line  (Fig.  12). 

45.  Alluvial  cones. — If  a  torrent  or  relatively  swift  stream 
discharges  into  an  open  valley  or  upon  a  plain,  it  will  build 


FIG.  25. — Alluvial  cone,  Visp  Valley, 


Photograph  by  the  author. 


at  the  mouth  of  its  gorge  a  fan-shaped  structure  to  which 
the  above  name  has  been  given.  It  will  be  but  the  segment 
of  a  cone,  having  its  apex  up  the  torrent  valley  and  its 
margin  extending  more  or  less  widely  on  the  plane  surface 
below.  The  deposit  is  due  to  the  loss  of  velocity  suffered 
by  the  stream  as  it  emerges  on  open  ground.  The  conical 
form  is  due  to  the  dropping  at  its  head  of  the  coarsest  and 


58  GEOLOGY 

most  abundant  materials,  and  to  the  frequent  shifting  of 
the  stream's  course.  Especially  in  powerful  floods  does  the 
stream  swing  from  one  side  around  to  the  other,  building 
up  in  irregular  succession  different  parts  of  the  cone.  If 
the  loss  of  velocity  be  very  great,  the  deposit  will  be  abrupt 
and  the  cone  steep.  If  the  change  be  moderate,  the  de- 
posit will  be  gradual  and  the  cone  broad,  gently  sloping, 
and  fanlike.  Sometimes  a  stream  cuts  a  deep  channel 
through  the  cone,  and  builds  a  new  one  at  a  lower  level, 
and  partly  between  the  dismembered  parts  of  the  older 
cone.  The  cone  may  form  a  barrier  extending  across  the 
main  valley,  making  a  lake  above,  or  may  be  built  into  a 
lake  occupying  the  valley,  turning  a  single  lake  into  two, 
as  at  Interlaken.  In  the  cases  last  described  the  cone,  or 
its  lower  part,  forms  also  a  delta.  In  some  mountain  re- 
gions alluvial  cones  attain  a  height  of  1,000  to  2,000  feet, 
and  an  extent  of  several  miles.  Davis  cites  the  fan  or  allu- 
vial cone  of  the  Mercer  Eiver,  which  issues  from  the  Sierra 
Nevada  upon  the  great  valley  of  California,  and  has  built  a 
fan  of  40  miles  radius. 

46.  Flood  plains. — As  we  cross  an  ordinary  open  valley, 
we  come,  at  the  foot  of  the  slope,  or  valley  wall,  upon  a 
level  tract  of  land,  wide  or  narrow,  on  one  or  both  sides  of 
the  stream.  It  is  mostly  composed  of  fine  soil  at  the  sur- 
face, though  if  we  dig  down  we  may  encounter  sand,  gravel, 
and  coarse  stones.  It  is  commonly  arable,  though  subject 
to  floods  in  the  spring,  or  at  other  times  when  the  channel 
is  too  small  to  hold  the  supply  of  water  from  the  higher 
grounds.  In  the  upper,  torrential  part  of  a  river  the  flood 
plain  does  not  appear,  or  is  rough  and  composed  of  such 
stones  and  bowlders  as  the  stream,  with  diminished  velocity, 
leaves  upon  its  banks.  Flood  waters  carry  much  fine  mat- 
ter gathered  from  the  slopes  lying  within  the  basin  of  the 
stream.  As  the  waters  spread  over  the  meadows  they  lose 
most  of  their  velocity  and  drop  a  sheet  of  silt  upon  the 
surface,  thus  raising  it  a  little  higher,  and  making  a  yearly 


60 


GEOLOGY 


or  more  frequent  contribution  to  its  fertility.  Emerson 
refers  to  the  farmers  who 

"  Thank  the  spring  flood  for  its  fertile  slime," 

and  the  Nile  and  many  other  illustrations  will  come  to  the 
mind  of  the.  student.  The  flood  grounds  of  the  Genesee  in 
western  New  York  are  in  some  places  nearly  two  miles 
wide.  The  Thames,  though  a  small  river,  has  so  widened 
its  valley  in  the  soft  rocks  above  Oxford  as  to  have  exten- 
sive flood  plains.  Those  of  the  lower  Mississippi  are  many 
miles  in  width.  The  Connecticut  above  Springfield,  the 
Mohawk  at  Utica,  the  Susquehanna  at  Williamsport,  and 
the  middle  Ehine  are  further  illustrations. 

The  course  of  a  stream  through  its  flood  grounds  is 
unstable.  Slight  obstacles  or  inequalities  in  the  resistance 
of  its  banks  are  enough  to  generate  strong  curves  and 
propagate  them  down  the  valley.  Such  curves  are  called 
meanders,  from  the  classic  river  of  Asia  Minor  which  illus- 
trates this  behavior  of  streams.  On  the  convex  side  of  such 
a  bend  the  river  cuts  away  its  banks,  and  a  bluff  with  cav- 
ing turf  and  denuded  roots  is  found.  Trees  of  considerable 
size  are  undermined  and  felled  in  this  manner.  On  the 
concave  side  of  the  bend  deposit  goes  on,  and  a  growing 
shingle  beach  slopes  gently  to  the  water.  The  sinuosity 
thus  increases  until  sections  of  the  stream  are  made  up  of 
a  series  of  oxbows.  The  neck  of  one  or  more  of  these  bows 
may  become  so  narrow  that  in  time  of  flood  the  river  cuts 
its  way  across  and  leaves  the  oxbow  at  one  side.  The  track 
of  the  steamboats  on  the  Mississippi  has  been  shortened 
18  miles  by  a  single  cut-off.  The  riverward  ends  of  the 
oxbow  silt  up,  being  at  the  point  where  moving  water 
comes  in  contact  with  still  water  and  drops  some  of  its 
load.  Thus  a  lagoon  is  formed.  A  subsequent  change  of 
course  by  the  river  may  leave  the  lagoon  at  some  distance, 
or,  in  a  large  valley,  many  miles  inland.  The  maps  issued 
by  the  Mississippi  River  Commission  give  many  examples 


GEOLOGY 


of  such  phenomena.  Mark  Twain's  accounts  of  the  insta- 
bility of  the  Mississippi  may  be  read  with  great  profit  in 
this  connection.* 

Portions  of  a  river  may  be  diverted  to  several  minor 
channels  upon  a  flood  plain   of   smooth   surface.      Thus 

the  road  leading  westward 
out  of  Oxford  crosses  the 
several  branches  of  the 
Thames  by  six  bridges. 
The  extent  of  such  wind- 
ings may  be  seen  in  the 
fact  that  the  distance  from 
the  mouth  of  the  Ohio  to 
the  Gulf  of  Mexico  is  500 
miles,  while  the  course 
of  the  river  covers  1,080 
miles. 

The  economic  signifi- 
cance of  flood  plains  is 
very  great.  Fourteen  per 
cent  of  the  arable  soil  in 
the  Eastern  United  States 
is  estimated  by  McGee  to 
be  of  this  alluvial  nature, 

and  25  to  30  per  cent  of  our  population  reside  upon  flood 
plains.  Protection  against  disaster  by  floods  is  therefore 
important,  and  it  is  to  be  obtained  by  avoidance  of  the 
lowest  grounds  for  building,  where  practicable,  by  substan- 
tial construction,  and  by  dikes  or  levees.  Within  recent 
years  the  swollen  Ohio  has  done  much  damage  in  the 
lower  parts  of  Cincinnati.  The  Khone  has  been  known 
to  rise  11^  feet  at  Lyons,  and  the  Seine  20  feet  at  Paris. 
In  the  autumn  of  last  year  (1899)  the  most  destructive 


FIG.  28.— Meanders  of  the  Missi 
with  cut  off  lakes. 


Life  on  the  Mississippi. 


RIVERS 


floods  since  1813  devastated  the  valleys  of  the  Isar  and  oth- 
er streams  of  Bavaria.  The  great  floods  of  the  Ganges  and 
the  Xile  are  well  known. 
The  most  important  flood 
problem  in  the  United 
States  is  that  of  the  low- 
er Mississippi,  where  vast 
areas  are  sometimes  flood- 
ed, and  protection  becomes 
a  question  of  national  in- 
terest. 

47.  Natural  levees.— 
"When  a  river  overflows 
its  immediate  banks  the 
course  of  the  water  is 
checked,  both  because  it 
becomes  shallow  and  be- 
cause it  encounters  the 
trees  and  shrubs  that  flour- 
ish in  such  places.  By 
reason  of  the  loss  of  ve- 
locity, much  of  the  load  is 
at  once  cast  down,  leaving 
a  thinner  mantle  of  fine 
silt  to  be  spread  over  the 
greater  part  of  the  flood 
plain  on  either  hand.  In 
this  manner  long  lines  of 
embankment  are  formed 
next  the  stream,  from 
which  the  land  slopes 

gently  back  to  the  valley  sides,  which  in  the  case  of  a 
great  river  like  the  Mississippi  may  be  several  miles  away. 
In  more  moderate  floods  these  natural  levees  may  be  seen 
between  the  main  channel  and  the  back  water  of  the 
meadows  and  fields.  This  phenomenon,  according  to  Lyell, 


FIG.  29.— Meanders  of  Brazos  River 
near  Granbury,  Texas. 


g4.  GEOLOGY 

appears  for  long  distances  on  the  Nile  in  the  time  of  the 
annual  flood. 

A  river  of  moderate  flow  carrying  a  full  load  will  also 
build  up  the  bottom  of  its  channel  as  well  as  its  banks,  and 
thus  may  flow  in  an  elevated  trench  of  its  own  making. 
The  Po  is  a' notable  illustration. 

48.  Terraces.— In  open  valleys,  or  along  the  "valley 
track "  of  rivers,  platforms  are  often  found  rising  like 
stairs  upon  the  slope.  They  are  of  variable  width,  and 
although  commonly  horizontal  to  the  eye  will  be  found  to 
descend  gently  with  the  stream.  Each  platform  may  be  a 
few  or  many  feet  high,  and  the  passage  from  one  up  to  the 
next  may  be  by  a  gentle  or  steep  incline.  They  may  appear 
in  pairs  on  opposite  sides  of  the  valley,  or  otherwise.  In 
material  and  appearance  they  are  like  flood  plains,  and  they 
commonly  are  the  remnants  of  old  flood  plains  which  have 
been  abandoned  by  the  river  as  it  has  deepened  its  valley. 
Each  higher  platform  or  terrace  is  in  this  case  older  than 
the  one  below  it.  In  course  of  time  the  present  flood  plain, 
whose  surface  is  raised  slightly  after  every  inundation,  will 
be  abandoned  by  the  river  if  the  latter  be  somewhat  swift 
and  still  eroding  its  bed.  Thus  another  terrace  or  pair  of 
terraces  will  be  added  to  the  series  and  a  new  flood  plain 
formed  at  a  lower  level.  The  interval  between  opposite 
terraces  is  wider  as  we  ascend,  and  the  distance  between 
the  upper  ones  may  be  many  times  the  width  of  the 
stream.  It  must  not  be  thought  that  the  stream  was 
necessarily  wider  when  anciently  flowing  at  the  higher 
level.  As  it  abandons  each  flood  plain,  the  river  not  only 
deepens,  but  widens  its  track  by  the  meandering  habit 
already  described,  thus  continually  encroaching  upon  the 
riverward  borders  of  the  level  it  has  left.  Hence  the  ter- 
races may  be  but  a  fraction  of  the  original  flood  plain. 
Such  encroachment  often  accounts  for  the  absence  of  a 
terrace  from  one  side  of  the  valley  when  it  is  present  on 
the  other. 


RIVERS 


65 


Terraces  may  consist  of  shelves  of  the  bed  rock  of  the 
region,  over  which  the  river  has  spread  a  mantle  of  gravel 
or  silt,  or  they  may  be  of  unconsolidated  land  waste  through- 
out. The  difference  depends  upon  the  history  of  theVal- 


Fio.  30.— Cross  section  of  valley  with  filling  and  subsequent  terracing,  a  a,  bed 
rocks;  bb,  upper,  older  terraces;  cc,  lower,  newer  terraces;  d,  low- water 
level  of  the  river. — After  SIIALER. 

ley.  In  the  latter  case  we  must  think  of  a  spacious  valley 
previously  excavated  in  the  rocks  and  afterward  filled  with 
waste.  The  stream  attacks  this  waste  and  carves  the  ter- 
races as  already  described.  Most  valleys  in  the  northern 
United  States  were  once  deeper  than  now,  but  during  the 
Glacial  period,  when  rock  destruction  was  rapid,  became 


FIG.  31.— Meanders  of  the  James  River  above  the  entrance  of  Appomattox  River. 

filled  sometimes  to  a  depth  of  several  hundred  feet.  Since 
that  time  the  streams  have  been  cutting  away  this  material, 
leaving  parts  of  it  as  terraces.  Kussell  cites  the  case  of 
Snake  Eiver,  the  largest  branch  of  the  Columbia.  A  canon 


QQ  GEOLOGY 

4,000  feet  deep  in  some  parts  has  been  excavated,  and  was 
filled  to  a  depth  of  360  feet  during  the  Glacial  time  with 
rubbish  worn  from  the  high  grounds  of  Idaho.  After  the  ice 
melted  away  the  river,  no  longer  overloaded,  turned  from 
deposition  to  excavation,  carrying  away  most  of  the  loose 
glacial  material,  and  cutting  into  the  rock  below.  The 
Merrimac  and  Connecticut  valleys  supply  more  familial- 
illustrations  of  such  terraces. 

The  platforms  here  described  are  called  alluvial  terraces, 
because  due  directly  to  river  action.  It  will  be  well  at  this 
point  to  note  certain  terraces  of  other  origin  which  might 
be  mistaken  for  them.  Thus  we  have  glacial  terraces 
formed  in  valleys  in  the  presence  of  waning  glaciers,  and 
terraces  of  differential  erosion,  due  to  the  outstanding  on  a 
hillside  of  horizontal  layers  of  harder  rock.  These  may  bet- 
ter be  called  benches.  Structures  resembling  river  terraces 
may  be  formed  on  the  shores  of  lakes  and  seas,  as  will  be 
explained  in  later  chapters.  If  a  valley  be  flooded  by  a  lake 
or  inflow  of  the  sea,  the  streams  which  enter  it  will  build 
deltas,  and  on  the  removal  of  the  water  by  evaporation  or 
emergence  of  the  land,  these  deltas  may  resemble  river  ter- 
races along  parts  of  the  valley.  Such  delta  terraces  are  com- 
mon in  the  Hudson-Champlain  and  Great  Salt  Lake  valleys. 

49.  Deltas. — A  delta  is  an  accumulation  of  land  waste 
at  the  mouth  of  a  stream,  on  the  border  of  a  lake  or  the 
ocean.  Its  surface  is  partly  a  land  area  and  partly  sub- 
merged. The  name  was  anciently  taken  from  the  sub- 
aerial  part,  because  the  Greeks  saw  that  these  deposits  at 
the  mouth  of  the  Nile  had  the  form  of  their  letter  delta  (A). 
They  did  not  take  account  of  the  outer  fringe  which  lies 
beneath  the  Mediterranean.  Deltas  are  due  to  the  loss  of 
velocity  which  a  stream  sustains  upon  entering  a  body  of 
water.  Its  load  is  laid  down,  the  coarser  fragments  at 
once,  and  the  fine  particles  after  traveling  some  distance 
from  land.  In  the  case  of  marine  waters,  deposition  is 
greatly  hastened  by  the  presence  of  salt  in  solution. 


RIVERS  67 

As  regards  deposition,  Russell  has  made  a  useful  dis- 
tinction between  the  deltas  of  high  and  low  grade  streams. 
If  a  stream  of  large  velocity  enters  a  body  of  water,  the 
load  is  at  once  dropped  and  an  embankment  is  thrown  out 
whose  surface  is  slightly  beneath  the  surface  of  the  water, 


Fio.  32. — Section  of  a  delta,  showing  inclined  beds  made  in  forward  growth, 
with  nearly  horizontal  beds  above.— After  GILBERT. 


and  whose  margin  descends  at  a  considerable  angle  to  the 
bottom.  A  low-grade  stream  entering  the  sea  after  a  long 
lowland  course,  like  the  Mississippi,  Is  ile,  or  Ganges,  carries 
nothing  but  fine  sediment,  loses  its  velocity  very  gradually, 
and  hence  builds  beneath  the  water  a  gently  sloping  plane 
or  fan,  all  of  whose  beds  depart  but  little  from  a  horizontal 
position. 

We  may  make  a  similar  distinction  between  the  land 
portions  of  deltas.  If  a  high-grade  stream  passes  directly 
from  its  torrent  valley  into  a  body  of  water,  its  delta  will 
be  a  partially  submerged  alluvial  cone.  Suppose,  however, 
that  a  long  valley  track  intervenes.  We  may  regard  the 
alluvial  cones  at  its  head  or  along  its  course  as  merging 
into  its  flood  plains,  and  these  in  turn  as  passing  into  the 
delta.  The  delta  begins  at  the  point  where  the  alluvial 
grounds  have  obviously  been  reclaimed  from  the  water 
body.  Likewise  the  natural  levees  continue  into  the  delta 
area,  the  instability  of  the  stream  increases,  and  branches 
are  given  off  to  find  the  sea  or  lake  by  independent  mouths. 
Such  branches  have  been  well  called  distributaries,  and 
they  accentuate  the  triangular  form  of  the  landward  por- 
tion of  the  delta. 


68 


GEOLOGY 


Deltas  may  be  instructively  observed  as  formed  by  rills 
in  transient  pools.  After  the  water  has  soaked  away,  the 
radiating  shallow  channels  on  the  surface,  the  lobate  points 


PIG.  33.-Delta  built  into  a  lake. 


of  discharge,  the  frontal  slope,  and  the  fringe  of  finer 
mud  appear.  Deltas  of  considerable  size  were  thus  formed 
in  temporary  lakes  of  Glacial  time,  and  may  now  be  studied 
to  great  advantage.  The  student  should  seek  for  deltas  at 


RIVERS  69 

the  heads  and  along  the  borders  of  lakes.  Ithaca  and  Wat- 
kins  are  built  upon  deltas  at  the  head  of  Cayuga  and  Seneca 
Lakes,  upon  whose  borders  farther  north  numerous  points 
are  found,  triangular  areas  of  upland  rubbish,  now  used  for 
summer  homes.  Several  streams  may  enter  a  mountain  lake, 
their  deltas  grow  rapidly,  in  time  meet  one  another,  and  the 
area  of  the  lake  becomes  a  meadow.  In  large  lakes  this 
process  would  require  a  long  period,  even  with  rapid  for- 
mation of  deltas.  Thus  Lake  Geneva  is  now  45  miles  long. 
The  Rhone  river,  muddy  with  the  waste  of  Alpine  glaciers 
and  torrents,  enters  it  on  the  east,  and  has  built  a  delta 
20  miles  long,  the  last  mile  of  which  has  been  made  since 
Roman  days. 

Rivers  entering  the  sea  do  not  commonly  form  deltas  if 
the  tide  is  strong.  The  Ganges  is  an  exception,  making  a 
stupendous  deposit  in  the  face  of  powerful  tides.  This  is 
no  doubt  due  to  its  great  volume,  and  to  the  mass  of  waste 
which  it  brings  down  from  the  highest  mountains  of  the 
world.  Also,  if  the  land  has  been  submerged,  and  the 
rivers  enter  a  tidal  sea  through  deep  channels,  the  river 
deposits  can  not  withstand  the  inrush  of  the  tides.  The 
Mississippi,  the  Nile,  the  Rhone,  the  Po,  and  the  Rhine 
form  deltas  in  regions  of  imperceptible  or  moderate  tides. 
That  the  Rhine  builds  a  great  delta,  and  the  Thames  enters 
the  sea  by  an  estuary,  is  probably  due  to  the  excessive  supply 
of  waste  which  the  Rhine  receives  from  its  high  sources, 
while  the  Thames  rises  in  an  area  of  slight  elevation,  drains 
a  small  area,  and  pursues  a  short  course  to  the  sea. 

We  may  now  briefly  review  the  extent  and  rate  of  growth 
of  some  of  the  great  deltas.  The  delta  of  the  Mississippi 
River  is  counted  to  begin  with  its  first  distributary  south 
of  the  Red  River,  and  its  area  is  somewhat  more  than  12,000 
square  miles.  Borings  show  that  the  alluvium  of  the  delta 
is  more  than  1,000  feet  deep  near  Xew  Orleans.  The  delta 
of  the  Yukon  has  a  length  of  100  miles,  and  a  seaward  mar- 
gin of  70  miles. 
6 


70 


GEOLOGY 


The  seaward  growth  of  the  Low  Countries  is  due  to  the 
building  of  great  deltas  by  the  Khine  and  the  Elbe,  mark- 
ing a  transfer  of  the  material  of  the  Alps  to  the  shallow 
seas  of  the  north.  The  ancient  records  of  the  presence  of 
civilized  man  on  the  shores  of  the  Mediterranean  make  the 


FIG.  34.-Delta  of  the  Mississippi  River. 


deltas  of  this  region  especially  instructive.  The  Rhone 
delta  has  grown  13  miles  since  the  beginning  of  the  Chris- 
tian era.  The  classic  port  of  Adria  is  now  20  miles  in- 
land, arid  the  coast  for  100  miles,  from  Trieste  to  Eaven- 


RIVERS  71 

na,  has  in  the  same  period  extended  seaward  from  2  to  20 
miles. 

50.  Estuary  deposits. — The  wide  mouths  or  lower  sec- 
tions of  tidal  rivers  are  called  estuaries.     There  is  in  them 
a  conflict  between  the  outflowing  fresh  waters  and  the  in- 
rushing  tide.     At  low  tide,  or  in  seasons  of  powerful  floods, 
the  river  may  have  the  advantage,  pushing  its  waters  and 
load  of  waste  seaward.     At  other  times  the  land  waste  is 
thrust  back,  and  marine  muds  from  adjacent  sea  bottoms 
are  carried  more  or  less  up  the  channel.     Such  channels, 
deep  enough  to  admit  the  sea,  are  commonly  due  to  a 
former  uplift  of  the  land  and  the  creasing  of  the  coast 
region  with  land  valleys  during  the  period  of  emergence. 
Subsequent  sinking  of  the  land  would  "  drown  "  the  lower 
parts  of  the  rivers.     The  deposits  of  an  estuary  tend  to  be 
cross-bedded  and  tumultuous,  owing  to  the  irregularity  and 
conflict  of  currents.     They  may  be  in  part  coarse,  if  the 
streams  head  in  a  glacial  or  mountain  region,  and  soon  pass 
into  the  estuary.     Some  deposits  are,  however,  of  fine  mud, 
much  shifted  and  finely  ground,  moved  up  and  down  at 
intervals  for  a  long  time,  before  escaping  into  the  sea.     The 
fine  ooze  off  the  piers  of  the  North  River  and  the  muds  of 
the  Thames  at  London  afford  illustrations.     Nearly  all  the 
rivers  of  our  Atlantic  coast  enter  the  sea  through  estuaries. 
Such  are  the  Hudson,  the  Delaware,  the  Susquehanna,  and 
the  Potomac.     The  Amazon  and  La  Plata  are  in  the  same 
class,  and  the  greater  rivers  of  England  owe  their  commer- 
cial importance  to  this  feature. 

51.  Bars. — These  are  closely  related  to  estuarine  depos- 
its.    Where  the  waters  of  a  river  are  checked  by  the  in- 
ward impulse  of  the  tide,  sediments  are  deposited,  making 
shoals  or  islands.     These  constitute  an  inner  bar.     Thus  in 
the  Hudson  River,  the  rise  of  the  tide  is  noted  as  far  as 
Albany  and  Troy.     For  some  miles  below,  sediments  accu- 
mulate rapidly,  being  checked   after  their  swift   descent 
from  the  uplands,  and  much  dredging  is  needed  to  keep 


72  GEOLOGY 

the  channel  open  for  steamships.  An  inner  bar  may  form 
at  any  point  where  velocity  is  diminished  and  much  sedi- 
ment is  deposited.  Outside  of  the  mouth  of  the  rivers  in 
the  margin  of  the  sea  bars  are  often  formed.  These  are 
extended  by  currents  moving  alongshore,  and  may  front 
much  of  a  coast  line,  as  is  the  case  with  New  Jersey  and 
the  Carolinas.  It  is  recorded  that  the  apostle  Paul  sailed 
up  the  Cestra  to  Perga  in  southern  Asia  Minor.  This 
would  now  be  impossible,  because  the  mouth  of  the  stream 
is  blockaded  by  an  outer  bar  formed  since  the  first  cen- 
tury A.  D. 

52.  Variable  constitution  and  structure  of  rocks, — A  river 
and  its  branches  sinking  their  channels  in  rocks  of  differ- 
ent degrees  of  hardness  and  in  beds  of  varying  position 
undergo  important  changes  of  direction  or  relation,  as  do 
different  rivers  or  systems  in  relation  to  each  other.     Some 
account  of  these  changes  will  be  given  in  the  section  on 
Physiographical  Geology.     It  is,  however,  necessary  at  this 
point  to  study  the  origin  of— 

53.  Waterfalls  and  rapids.— If  a  stream  flows  from  harder 
upon  softer  beds,  the  latter  will  be  cut  away  quickly  and  a 
rapid  will  result.     Eivers  thrown  out  of  their  ancient  course 
by  glacial  accumulations  often  encounter  masses  of  hard 
rock  in  sinking  their  new  channel.     They  cut  away  the 
loose  drift  below  and  plunge  swiftly  over  the  unyielding 
barrier.    The  water  power  of  Lowell,  Mass.,  Pawtucket,  R.  I., 
or  of  Waterbury,  Conn.,  results  from  such  changes.     The 
rivers  of  the  Atlantic  slope  south  of  New  York  flow  from 
hard  older  rocks  upon  soft  newer  rocks,  and  form  thus  a 
series  of  rapids  and  falls,  at  what  is  called  the  Fall  Line, 
on  which   Trenton,  Richmond,  Raleigh,  and  other  cities 
have  grown  up.     Bowlders  and  coarse  rubbish  brought  into 
the  channel  of  a  trunk  stream  by  the  torrential  current  of 
a  tributary  may  form  a  barrier  and  occasion  rapids. 

Waterfalls  are  often  formed  by  the  recession  of  horizon- 
tal beds  of  unequal  power  of  resistance.     If  a  hard  layer 


FIG.  35.  — Succession  of  waterfulls  over  horizontal  beds  of  limestone, 
Trenton  Falls,  N.  Y. 


74  GEOLOGY 

lies  over  a  soft  bed,  the  latter  is  cut  and  weathered  away 
more  rapidly,  leaving  the  former  as  a  projecting  shelf,  over 
which  the  water  plunges.  The  blows  of  the  water  and  the 
continued  moisture  break  up  the  soft  under  rocks,  while 
portions  of  the  hard  cap  fall  down  from  time  to  time  as 
the  support  is  removed.  Illustrations  of  this  process  on  a 
small  scale  may  be  seen  in  numberless  ravines.  Niagara  is 
formed  on  the  same  principle.  When  the  river  began  to 
take  its  present  course,  it  found  a  high  bluff  known  as  the 
Niagara  Escarpment,  crossing  its  course  toward  Lake  Onta- 
rio. In  this  region  a  mass  of  hard  limestone  overlies  soft 
shales.  In  the  manner  already  described  these  rocks  have 
been  cut  away  and  the  falls  have  moved  southward  7  miles, 
leaving  a  gorge  of  that  length.  With  this  brief  reference 
to  the  principle  on  which  Niagara  was  formed,  we  defer 
the  fuller  study  of  its  history  to  the  chapter  on  the  Glacial 
Period.  The  Falls  of  the  Genesee  at  Rochester  and  Port- 
age are  formed  thus,  as  also  are  the  Falls  of  St.  Anthony 
at  Minneapolis.  Shoshone  Falls  in  Idaho  have  a  like  ori- 
gin, though  the  beds  in  this  case  are  lavas  of  extinct  vol- 
canoes. 

The  presence  of  vertical  joints  facilitates  the  formation 
of  waterfalls  in  this  manner,  and  small  falls  are  often 
formed,  due  to  this  cause,  when  there  is  no  apparent  dif- 
ference in  the  hardness  of  the  upper  and  lower  beds. 
Dikes  of  hard  rock  may  cross  the  track  of  a  river  and 
cause  waterfalls.  Such  cases  are  found  in  the  Cascade 
region.  The  recession  would  here  be  limited  to  the  thick- 
ness of  the  dike.  When  this  is  worn  away  a  rapid  will 
ensue,  and  finally  a  well-graded  stream  bed. 

A  canon,  rapidly  or  effectively  deepened  by  stream 
action  or  formerly  existing  glaciers,  may  receive  feeble 
tributary  streams  which  are  unable  to  sink  their  channels, 
and  which  therefore  plunge  over  precipices  of  great  height. 
Such  are  some  of  the  "  bridal  veil "  falls  as  in  the  Yosem- 
ite,  the  slender  thread  of  water  breaking  into  foam  in  its 


RIVERS  75 

long  descent.     Numerous  splendid  examples  are  also  found 
in  the  Alps. 

54.  Changes  due  to  climate. — The  Mississippi  River  has 
its  sources  in  regions  of  varying  temperature  and  rainfall, 
and  is  therefore  fairly  constant  in  its  flow  in  different  parts 
of  the  year,  with  now  and  then  an  exceptional  flood.     The 
Nile,  however,  rising  in  a  tropical  region,  is  much  affected 
by  the  rainy  season,  and  is  subject  to  a  periodical  flood. 
Eivers  may  flow  copiously  from  mountains,  as  in  the  west- 
ern United  States,  but  in  entering  more  arid  districts,  in 
western   Nebraska  and   Kansas,  lose   by  evaporation  and 
soakage,  until  the  stream  bed  is  left  dry  or  with  but  scat- 
tered pools,  during  much  of  the  year.     In  a  similar  manner 
the  Abana  and  Pharpar,  "  rivers  of  Damascus,"  rise  in  the 
copious  springs  of  Anti-Lebanon,  and  lose  themselves  in  the 
desert  east  of  the  city  whose  life  and  beauty  they  create. 

55.  Scenic  and  economic  significance  of  rivers. — Such  con- 
siderations belong  especially  to  the   domain  of  physical 
geography,  but  here  demand  brief  attention.     In  opening 
up  the  continents  to  man,  rivers  and  their  valleys  have 
always  been  the  avenues  of  discovery  and  settlement.    Here 
first  forests  are  cleared,  roads  built,  crops  planted,  and  cities 
founded.     With  few  exceptions  inland  cities  stand  where 
rivers  have  conferred   upon  early  settlers    some    special 
advantage.     Nearly  all    available   water  power  is   due  to 
changes  in  ancient  streams,  by  which  rapids  and  waterfalls 
have  been  formed.     A  multitude  of  cases  could  be  cited 
from  New  England  and  many  northern  regions.     Most  im- 
portant engineering  problems  and  public  works  have  to  do 
with  the  navigation  of  rivers  and  the  averting  of  floods. 
The  usefulness  of  rivers  is  dependent  upon  the  preserva- 
tion of  forests,  and  the  Old  World  is  promptly  followed  by 
the  New  in  attention  to  this  branch  of  practical  science. 
The  beauty  of  rivers  is  a  theme  for  every  lover  of  Nature, 
and  opens  endless  fields  for  the  higher  appreciation  of  the 
world  in  which  we  live. 


CHAPTER  IV 

UNDERGROUND   WATERS 

56.  Water  in  all  rocks. — The   mineral  constituents  of 
rocks  are  not  so  closely  aggregated  as  to  prevent  the  admis- 
sion of  water.     According  as  the  rock  has  a  close  or  open 
texture  it  will  admit   water  in   small  or  large  amounts. 
Water  may  circulate  with  more  or  less  freedom  in  rocks 
that  lie  above  the  sea  level.     Wherever  soluble  minerals, 
like  rock  salt,  are  undisturbed,  it  is  evident  that  there  has 
been  little  movement   of   subterranean  waters.     In  com- 
pact rocks,  or  in  those  lying  below  sea  level,  the  water  con- 
tained may  be  that  of  original  deposition — that  is,  it  was 
involved  with  the   sediments  forming  the  rock  as  they 
were  laid  down  in  ancient  seas.     Hence  such  water  is  often 
found  to  contain  salt.     The  more  ancient,  compact,  crystal- 
line rocks  contain  about  0.06  per  cent  of  water.     The  ordi- 
nary sedimentary  rocks  average  2.05  per  cent,  though  the 
amount  is  very  variable.     Gravels,  sands,  and  clays  may 
contain  as  much  as  10  per  cent.     Dana  estimates  that  there 
is  water  enough  in  the  underlying  rocks  of  the  earth's  crust 
to  make  a  layer  1,300  feet  deep  over  the  continents.     This 
is  sufficient  to  produce  most  important  chemical  and  me- 
chanical effects,  to  which  we  now  turn. 

57.  Oxidation. — This  effect  of  water  has  already  been 
considered  under  Weathering  (see  §  14) ;  but  reference  was 
there  had  only  to  superficial  rocks  and  soils.     It  is  evident 
that  along  porous  beds  of  rock  which  are  inclined  to  the 
surface,  as  well  as  through  fissures,  water  may  descend  to 

76 


UNDERGROUND  WATERS  77 

great  distances  and  aid  in  producing  decay.  Especially  is 
this  true  in  regions  of  recent  lavas  where  much  heat  re- 
mains. 

58.  Solution  and  cementation. — Solution  has  also  been 
noticed  as  a  superficial  process.     This  may  go  on  to  a  depth 
of  some   thousands   of  feet.     We  have  seen  that  surface 
water  carries  down  with  it  various  solvent  acids.     By  their 
operation  nearly  all  minerals,  in  small  or  large  measure,  are 
eaten  away,  and  the  dissolved  matter  is  redeposited  at  lower 
levels.     To  this  process  is  largely  due  the  consolidation  of 
rocks.     Muds  are  turned  into  shales,  sands  become  sand- 
stones, and  gravels  are  bound  into  conglomerates.     Certain 
beds  or  pockets  of  glacial  sands  are  often  changed  into 
resistant  rock,   while  the   adjacent   masses    can    be  still 
removed  with  a  shovel.     The  more  common  cement  is  car- 
bonate of  lime,  but  iron,  silica,  and  other  substances  act  in 
the  same  manner.     The  most  striking,  though  not  the  most 
important,  effect  of  solvent  water  is  in  the  excavation  of  cav- 
erns.   As  the  phenomena  of  caverns  involve  several  process- 
es, both  chemical  and  mechanical,  constructive  and  destruc- 
tive, they  are  treated  at  a  later  point  in  this  chapter. 

59.  Deposition  in  pockets  and  fissures. — Any  such  openings 
are  likely  to  be  filled  by  means  of  descending  water,  rede- 
positing  matter  from  solution.     This  process  is  akin  to  the 
last  described.     As  it  is  also  one  mode  of  vein-making,  it 
will  be  noticed  again  in  Part  II.     Here,  as  so  often,  a  dy- 
namic process  leads  to  a  definite  form  or  structure.     The 
two  belong  together,  and  are  given  separate  treatment  only 
as  a  matter  of  convenience. 

GO.  Mechanical  erosion  by  underground  waters. — When  a 
channel  large  enough  to  allow  the  free  movement  of  water 
has  in  any  manner  been  formed  below  the  surface,  streams 
may  gather  their  abrasive  tools,  and  carry  on  destructive 
work  as  effectively  as  if  flowing  in  the  open  air.  It  is  clear 
that  such  work  can  only  be  done  above  the  level  of  the  sea, 
or  above  the  horizon  of  the  surface  drainage  to  which  the 


Yg  GEOLOGY 

subterranean  stream  is  tributary.     We  shall  observe  this 
process  again  in  our  study  of  caverns. 

61.  Landslides. — We  can  make  no  hard-and-fast  distinc- 
tion between  the  creeps  described  under  Weathering  and 
the  more  massive  and  deep-seated  movements  to  which  we 
now  refer.  '  The  latter  may  occur  in  several  ways.  If  a 
mass  of  soil  or  coarser  debris  be  insecurely  poised,  as  on  a 
steep  slope,  and  it  becomes  water-soaked,  the  friction  that 
held  the  particles  together  is  diminished,  and  the  entire 


PIG.  m.-Debris  of  landslide  of  1806,  Goldau,  Switzerland. 
Photograph  by  the  author. 

mass  may  push  downward  in  a  precipitate  and  confused 
way,  forming  an  irregular  group  of  knolls  and  hills  at  the 
bottom.  Within  the  memory  of  white  settlers,  17  acres  of 
land  thus  slid  from  the  steep  western  slope  of  the  Genesee 
valley  into  the  Gardeau  Flats  below  Portage.  If  beds  of 
rock  incline  with  the  slope  of  hill  or  mountain,  under-beds, 
as  of  clay,  may  become  moistened  with  percolating  water, 
and  serve  as  a  lubricant  on  which  the  overlying  masses  slip 
down  into  the  valley.  Similarly,  water  passing  through 
porous  layers  may  carry  out  much  matter  in  solution,  and 
bring  down  the  parts  above  by  undermining.  If  steep  cliffs 
are  undercut  by  rivers  or  wave  action,  large  portions  may 
readily  fall  off,  especially  if  the  rocks  be  vertically  jointed. 
Thus  rock  falls  of  tremendous  proportions  occur. 


UNDERGROUND  WATERS  79 

It  is  evident  that  the  sides  of  valleys,  particularly  among 
high  mountains,  and  the  steep  shores  of  lakes  and  oceans, 
are  the  theaters  of  such  movements.  In  addition  to  the 
case  above  given,  Dana  cites  a  destructive  earth  slide  occur- 
ring in  the  White  Mountains  in  1826,  and  Davis  describes 
a  great  slide  in  the  upper  valley  of  the  Ganges,  in  which, 
in  three  days,  800,000,000  tons  of  rock  fell,  leaving  a  cliff 
thousands  of  feet  high  as  a  scar  on  the  mountain  slope,  and 
throwing  a  barrier  1,000  feet  high  across  the  valley.  The 
bursting  of  the  lake  which  thus  formed  was  foreseen,  and 
the  immense  flood  that  followed  was  not  attended  with  loss 
of  life.  Destructive  slides  of  vast  extent  have  occurred 
among  the  Alps,  sometimes  destroying  entire  villages. 
Similar  catastrophes  have  no  doubt  happened  in  great  num- 
bers before  man  lived  on  the  earth.  Moderate  slides  and 
falls  are  constantly  occurring  in  all  regions  of  rugged  relief, 
so  that  this  form  of  geological  activity  assumes  high  im- 
portance in  the  history  of  the  earth.  Further  reference 
should  briefly  be  made  to  landslides  on  seashores.  Thus 
the  English  geologists  have  given  many  records  of  such 
"  founders "  on  the  coast  of  Devon  and  Dorset  in  south 
England.  The  rocks  dip  toward  the  sea,  and  clays  are 
covered  by  firmer  strata,  as  above  described.  Twenty-two 
acres  of  land  were  involved  in  one  of  these  movements. 
Slides  of  small  proportions,  but  of  serious  consequences, 
often  occur  along  railway  cuttings  whose  angle  of  slope  is 
too  great  for  security. 

62.  Springs. — These  are  formed  by  underground  waters 
outpouring  at  the  surface,  and  arise  in  several  ways.  Water 
sinking  through  soils  and  gravels  may  encounter  a  surface 
of  the  hard  rock  down  which  it  flows  to  escape  at  some 
lower  point,  as  the  base  of  a  hill.  Water  entering  porous 
or  jointed  rocks  may  sink  until  it  reaches  a  less  porous 
bed,  along  which  it  passes  to  some  point  of  outcrop.  This 
process  is  facilitated  if  the  beds  be  tilted  at  a  considerable 
angle,  and  the  waters  of  the  spring  may  boil  up  vigorously, 


80 


GEOLOGY 


because  of  the  hydraulic  pressure  behind  and  above.  Hence 
the  abundance  and  strong  flow  of  springs  in  mountain 
regions,  where  dislocations  permit  free  entrance  of  surface 
waters,  and  sloping  beds  favor  their  outflow  at  lower  levels. 
The  water  of  springs  is  commonly  cool  and  clear,  because 
of  its  passage  through  regions  little  affected  by  the  sun's 
heat,  and  because  slowly  moving  underground  streams  can 
carry  little  rocky  material  in  suspension.  Some  springs 
attain  great  volume.  In  the  Xittany  Valley  at  Bellefonte, 
Pa.,  a  spring  rises  whose  flow  is  about  14,000  gallons  per 
minute.  It  is  a  region  of  dislocations,  and  the  limestones 
which  form  the  uneven  floor  of  the  great  valley  have  become 
cavernous,  receiving  the  surface  waters,  to  emit  them  here 
and  there  in  powerful  springs.  Silver  Spring  in  Florida,  at 
the  head  of  the  Ocklawaha  River,  is  said  by  Le  Conte  to 
send  forth  such  abundant  waters  that  small  steamers  ascend 
the  river  and  enter  the  pool  of  the  spring.  Another  illus- 
tration is  found  at  the  base  of  Mount  Hermon,  in  the  great 
spring  which  supplies  the  head  waters  of  the  Jordan.  If 
the  structure  of  the  underlying  beds  favors  it,  fresh  water 
may  flow  for  some  distance  under  the  sea,  and  rise  in  the 
form  of  springs  through  the  shallow  marginal  waters.  In  a 
similar  manner  lakes  are  often  fed  by  streams  rising  from 
the  bottom. 

63.  Mineral  springs. — All  ground  water  dissolves  min- 
erals, and  might  strictly  be  called  mineral  water.  But  we 
reserve  the  term  for  waters  which  have  a  considerable  quan- 
tity of  dissolved  mineral  matters,  particularly  of  such  as 
give  them  medicinal  value.  It  is  plain  that  the  character 
of  the  water  will  depend  on  the  chemical  constituents  of 
the  rocks  through  which  it  has  passed.  Thus  some  spring 
waters  abound  in  sulphur,  in  iron,  and  in  various  com- 
pounds of  sodium,  potassium,  calcium,  and  magnesium. 
Some  famous  springs  are  determined  by  fractures  or  dislo- 
cations, permitting  free  rise  of  the  waters.  Such  are  those 
of  Saratoga,  rising  along  the  line  of  a  fault,  which  is  marked 


UNDERGROUND   WATERS  81 

by  a  shallow  valley  running  north  and  south  through  the 
town.  A  publication  of  the  United  States  Survey  reports 
about  10,000  mineral  springs  in  this  country,  and  the  list  is 
no  doubt  very  incomplete. 

04.  Thermal  springs. — Spring  water  may  not  feel  warm 
to  the  hand,  but  receives  the  above  designation  if  it  have 
a  higher  temperature  than  that  of  the  region  in  which  it 
flows.  If  it  descends  to  deep  levels  and  returns,  it  may  be 
warmed  by  the  remaining  heat  of  the  earth's  interior.  Fre- 
quently, however,  waters  are  heated  by  contact  with  buried 
lavas,  which,  though  relatively  near  the  surface,  have  not 
yet  cooled.  Under  this  head  belong  geysers,  which,  how- 
ever, will  be  treated  in  the  chapter  on  Volcanic  Action. 
Among  the  best-known  thermal  waters  in  the  United  States 
are  those  of  Hot  Springs,  Ark.,  and  many  springs  in  the 
Yellowstone  Park.  Others  are  at  Glenwood,  Col.,  where  a 
number  of  currents  of  very  hot  water  boil  up  in  and  near 
the  channel  of  the  Grand  River.  Accounts  of  the  Corn- 
stock  mine  in  Xevada  vividly  describe  the  difficulties  en- 
tailed upon  the  miners  by  the  scalding  waters  of  the  lower 
levels.  There  is  a  great  hot  spring  at  Bath,  England,  occa- 
sioning a  sanitary  resort  since  Roman  times.  The  tempera- 
ture ranges  from  104°  to  120°  F.,  and  the  discharge  is  385,000 
gallons  daily.  The  depth  of  the  sources  has  been  com- 
puted at  3,500  feet.  Owing  to  the  greater  solvent  powers  of 
heated  water  such  springs  carry  much  mineral  matter. 

65.  Deposits  from  springs. — As  water  emerges  at  the 
surface  it  loses  heat,  or  is  relieved  from  pressure,  and  con- 
sequently its  power  to  retain  minerals  in  solution  is  dimin- 
ished, and  deposition  takes  place  about  the  spring  and 
along  the  stream  which  flows  from  it.  Very  commonly 
these  deposits  are  of  lime  carbonate  dissolved  from  the 
limestones  traversed  by  the  flow.  It  forms  a  porous  incrus- 
tation, often  coating  twigs,  leaves,  and  mosses,  thus  preserv- 
ing their  forms.  Such  a  deposit  is  called  calcareous  tufa,  or, 
if  concretionary,  travertine,  and  is  often  seen  about  the  base 


g2  GEOLOGY 

of  limestone  hills.  Accumulations  of  one  foot  in  thickness 
in  four  months  are  reported  from  Tuscany,  now  forming  a  hill 
250  feet  in  height.  Springs  may  form  a  brownish  deposit  of 
iron,  and  more  rarely  silica,  or  siliceous  sinter,  as  it  is  called, 
is  laid  down.  This  happens  only  in  rare  conditions  under 
which  the  usually  insoluble  silica  can  be  affected.  Such 
deposits  of  importance  are  found  in  the  Yellowstone  Park, 
in  New  Zealand,  and  in  Iceland — all  geyser  regions.  In  the 
first  of  these  localities  the  deposit  is  largely  due  to  lowly 
organisms  (algae),  which  secrete  silica  from  the  waters. 

66.  Wells.— Ground  water  is  formed  at  varying  horizons, 
according  to  the  nature  of  the  soil  and  hard  rocks,  which 
thus  determine  the  necessary  depth.  In  sandy  soils  an 
abundant  flow  is  often  found  but  a  few  feet  below  the  sur- 
face. Such  water  is  quite  sure  to  be  contaminated  and  unfit 
for  use,  if  in  a  town  or  closely  settled  region.  Drainage 
from  the  stockyard  often  defiles  the  wells  about  farmers' 
dwellings.  The  surface  of  the  ground  may  slope  in  one 
direction,  while  the  dip  of  the  layers  of  rock  or  sand  is 
such  as  to  carry  sewage  directly  into  wells  whose  opening 
may  be  at  a  higher  point.  Well  water  in  towns,  no  matter 
how  clear  in  appearance,  can  safely  be  counted  dangerous. 
Only  expert  analysis  should  be  trusted,  and  the  test  must 
be  often  repeated.  If  a  well  is  sunk  below  a  bed  of  fine 
clay,  and  securely  cased  down  to  the  clay,  its  waters  are 
likely  to  be  pure.  The  contamination  of  surface  and  sub- 
terranean waters  in  populous  regions  leads  more  and  more 
to  wise  care  and  large  expenditure  for  a  trustworthy  supply. 
We  may  cite  in  illustration  the  large  attention  given  to 
water  supply  by  the  State  of  New  Jersey,  especially  for  the 
dense  populations  near  New  York  ;  also  the  care  bestowed 
upon  the  Croton  watershed,  and  the  conducting  of  Lake 
Skaneateles  waters  at  great  expense  to  the  city  of  Syracuse, 
and  of  the  Thirlmere  waters  to  Manchester.  Nor  should 
we  omit  the  growing  recognition  of  the  necessity  of  outside 
water  supply  in  all  villages  of  any  size. 


UNDERGROUND  WATERS  83 

Artesian  wells  are  so  called  from  the  province  of  Artois 
in  France,  where  this  method  of  obtaining  water  has  long- 
been  used.  In  a  true  artesian  well  the  water  flows  freely, 
or,  if  the  pressure  be  sufficient,  forms  a  jet  or  fountain. 
The  conditions  are :  The  existence  of  a  slightly  inclined 
and  continuous  porous  stratum  lying  between  compact  or 
impervious  strata  above  and  below.  The  latter,  which  may 


Flo.  37.— Ideal  section  illustrating  the  chief  requisite  conditions  of  artesian  wells. 
A,  a  porous  stratum  ;  B  C,  impervious  beds  below  and  above  A,  acting  as  con- 
fining strata ;  F,  the  height  of  the  water  level  in  the  porous  bed  A,  or,  in  other 
words,  the  height  of  the  reservoir  or  fountain  head  ;  D  E,  flowing  wells  spring- 
ing from  the  porous  water-filled  bed  A.— After  CHAMBEKLIN. 

be  of  clay  or  any  fine-grained  rock,  serve  to  confine  the 
water.  The  porous  bed,  as  of  sand  or  sandstone,  serves  to 
conduct  the  water  from  the  surface  and  also  as  a  reservoir. 
If  a  boring  be  made  from  the  surface  in  the  direction  of 
the  "  dip,"  by  the  ordinary  principle  of  hydraulic  pressure, 
a  flowing  well  is  produced.  The  inclination  of  the  beds  is 
usually  slight,  but  a  few  feet  to  the  mile,  and  the  well  is 
often  some  scores  or  hundreds  of  miles  from  the  point 
where  the  waters  enter  the  earth.  It  is  evident  that  a 
region  of  much-broken  or  dislocated  rocks  is  unfavorable 
for  artesian  wells.  Such  sources  of  water  supply  are  im- 
portant along  the  Atlantic  coast,  as  in  southern  New  Jer- 
sey. There  are  many  artesian  wells  in  the  valley  of  the 
Mississippi  and  in  the  region  of  the  Great  Lakes.  Arte- 
sian waters  have  long  been  used  in  Chicago,  the  wells  being 
supplied  from  the  gently  inclining  rocks  of  southern  Wis- 
consin. Other  wells  are  found  at  Louisville,  St.  Louis,  and 
New  Orleans.  They  are  important  on  the  Great  Plains,  in 
Dakota,  Nebraska,  and  Colorado.  Utah  and  California  are 
other  localities,  the  waters  serving  for  domestic  use  and  for 
irrigation. 


GEOLOGY 


CAVERNS 

67.  Classes  of  caverns. — Open  spaces  carried  to  greater 
or  less  distance  within  the  rocks  may  be  produced  in  sev- 
eral ways.     Thus  we  have  sea  caves  hewn  out  by  the  waves. 
These  are  never  of  great  size.     Also  along  planes  of  dislo- 
cation when  fissures  are  made  and  parts  of  the  rocky  crust 
of  the  earth  move  upon  each  other  small  cavernous  open- 
ings may  be  formed.     The  outflow  of  lavas  from  a  cooling 
crust  sometimes  gives  origin  to  caves.    But  the  great  caves 
are  always  due  to  the  circulation  of  waters  beneath  the 
earth's  surface,  and  their  mode  of  formation  will  now  be 
described. 

They  are  chiefly  due  to  processes  of  solution  and  must 
therefore  be  made  in  rocks  which  yield  to  the  attack  of 
water  and  the  acids  which  water  carries.  The  only  com- 
mon rock  of  which  this  is  true  is  limestone,  and  therefore 
it  is  in  such  rock  that  all  great  caverns  are  excavated. 

68.  Conditions  and  mode  of  formation.— For  the  making 
of  great  caverns  it  is  essential  that  the  limestone  consist  of 
massive,  thick  beds,  comparatively  free  from  alternating 
layers  of  sandstone  or  shale.     Along  the  downward  line 


FIG.  38 — Diagram  showing  action  of  poil  water  in  excavating  caverns.  A  A,  layers 
of  limestone  easily  dissolved  in  soil  water ;  B  B,  sink  holes  by  which  soil  water 
enters  the  cave  ;  C  (7,  vertical  shafts  or  domes  ;  DD,  horizontal  galleries.  At  the 
right  is  a  natural  bridge,  or  remnant  of  a  large  cave.— After  SHALER. 

formed  by  the  intersection  of  two  joint  planes  the  waters 
slowly  find  their  way.  They  are  laden  with  acids  gathered 
from  the  air  and  from  vegetation,  and  solution  takes  place, 
slowly  forming  what  are  termed  sink  holes.  From  these 
the  process  of  solution  works  laterally  along  the  planes  of 


UNDERGROUND  WATERS  85 

bedding,  and  often  at  several  levels.  Thus  a  region  may 
become  honeycombed  with  a  network  of  vertical  and  hori- 
zontal passages.  The  entrance  may  be  small  and  long 
undiscovered,  and  yet  lead  to  extended  tunnels  and  lofty 
chambers.  The  waters  by  which  the  work  is  effected  find 
their  way  laterally  to  adjacent  hillsides  or  mountain  slopes 
and  issue  as  springs  or  brooks  to  unite  with  the  surface 
streams.  It  is  evident  that  such  movements  can  only 
take  place  above  the  drainage  level  of  the  region.  At 
lower  levels  water  could  circulate  but  slightly,  if  at  all.  A 
cavern  is  limited  in  depth  by  the  position  of  the  local  base 
level  of  erosion.  After  cavernous  passages  become  large 
enough  to  permit  the  free  flow  of  water,  they  may  then  be 
enlarged  and  deepened  by  mechanical  erosion.  Lakes, 
rapids,  and  waterfalls  are  also  found  in  caverns.  As  the 
excavation  progresses,  the  roofs  of  passages  which  lie  near 
the  surface  are  weakened  and  fall  in.  Thus  some  gorges 
originate,  and  wherever  portions  of  the  roof  remain,  the 
so-called  natural  bridges  are  formed,  as  the  famous  Natu- 
ral Bridge  of  Virginia. 

69.  Deposition  in  caverns. — Erosion  is  not  the  only  im- 
portant process  which  takes  place  in  subterranean  cham- 
bers. Minerals,  especially  carbonate  of  lime,  are  dissolved 
by  percolating  waters  and  redeposited  on  the  walls  and 
floors.  Thus  at  a  point  where  water  slowly  drips  from  the 
roof  of  the  cavern,  pendant  needlelike  masses  form,  which 
are  called  stalactites.  Sometimes  they  are  broad  and 
massive  rather  than  round  and  slender,  and  they  may 
attain  great  length  and  size.  They  steadily  grow  in  length 
downward,  and  by  accretion  outward,  in  successive  layers, 
so  that  a  cross  section  is  similar  in  general  appearance  to 
the  cross  section  of  a  twig  or  tree.  Xot  all  of  the  material 
in  solution  is  thus  deposited,  but  some  is  carried  by  the 
dripping  water  to  the  floor  of  the  cavern,  forming  irregu- 
lar, knobby  masses,  called  stalagmites.  Sometimes  the  sta- 
lactite grows  downward  and  becomes  continuous  with  the 
7 


PIG.  89.-Hemnant  of  underground  stream  channel,  Natural  Bridge  Va 
Photograph  by  U.  S.  Geological  Survey. 


UNDERGROUND   WATERS  87 

stalagmite.  At  some  points  the  roof  undergoes  disintegra- 
tion, and  fine  material  and  coarse  fragments  fall  to  the 
floor.  These  may  be  worked  over  or  mingled  with  waste 
brought  by  streams  from  the  surface.  Not  infrequently 
caves  have  been  the  refuge  or  home  of  animals  and  of 
primitive  savages,  and  thus  organic  remains  are  added  to 
the  rest,  even  those  of  historic  and  civilized  man  being 
found  in  some  countries.  Thus  the  processes  of  destruc- 
tion and  accumulation  go  on  underground  in  cavernous 
regions  as  variously  as  on  the  surface  of  the  earth. 

70.  Noteworthy  caverns. — One  of  the  most  famous  cav- 
erns of  the  Old  World  is  at  Adelsberg  in  Austria.  Tt  con- 
sists of  four  great  grottoes,  in  which  an  annual  festival  is 
said  to  have  been  held  on  Whitsunday.  It  was  known  in 
the  middle  ages,  afterward  fell  out  of  mind,  and  was  re- 
discovered in  1815.  The  Mammoth  Cave  of  Kentucky  is 
the  best  known  of  American  caverns,  having  been  exten- 
sively studied  and  carefully  mapped.  It  was  found  in  1809 
by  a  hunter  who  was  pursuing  a  wounded  animal.  It  is 
excavated  in  the  subcarboniferous  limestone,  having  its 
mouth  in  a  ravine  600  feet  above  the  sea.  There  are  150 
miles  of  passages,  with  lakes,  rivers,  and  a  waterfall  250 
feet  in  height.  The  extremity  of  the  cave  is  several  miles 
from  the  entrance.  It  has  been  estimated  that  there  are 
in  Kentucky  100,000  miles  of  subterranean  channels  suffi- 
ciently large  to  permit  the  passage  of  a  man.  Many  "  sinks  " 
are  found  upon  the  surface,  due  to  subsidence.  Some  of 
these  sinks  are  occupied  by  small  lakes.  Luray  Cavern,  in 
the  Shenandoah  Valley,  Virginia,  was  discovered  by  means 
of  such  a  sink  in  1878.  It  surpasses  in  the  number,  size, 
and  brilliant  coloring  of  its  stalactites,  of  which  40,000  are 
said  to  be  visible  from  a  single  point.  Of  these  are  the 
"  Swords  of  the  Titans,"  eight  in  number,  50  feet  long,  3  to 
8  feet  wide,  1  to  2  feet  thick  and  hollow.  Their  edges  are 
thin,  and  they  give  off  deep,  resonant  sounds  when  struck. 
Howe's  Cave,  in  eastern  New  York,  is  a  smaller  but  well- 


UNDERGROUND  WATERS  89 

known  limestone  cavern,  and  Wind  Cave  is  an  important 
illustration  of  cave-making  in  South  Dakota. 

Cave-making  is  of  much  geological  importance.  We 
shall  learn  that  during  geological  eras  large  bodies  of  lime- 
stone have  not  seldom  been  removed  by  erosion.  A  consid- 
erable proportion  of  such  erosive  work  has  always  been  sub- 
terranean. The  peculiar  environment  which  caverns  afford 
for  animal  life,  as  in  the  case  of  fishes  and  insects,  has  pro- 
duced changes  of  structure  of  high  interest  to  the  student 
of  biology.  Mythology  and  ancient  history  abound  in  ref- 
erences to  caverns  in  relation  to  man,  who,  as  in  the  nar- 
ratives of  the  Hebrew  Scriptures,  used  them  as  places  of 
habitation,  refuge,  and  of  burial. 


CHAPTEK  V 

GLACIERS 

71.  Definition. — A  glacier  is  a  mass  of  ice  flowing  in  a 
valley  or  overspreading  a  tract  of  country.  The  size,  form, 
and  behavior  of  glaciers  depend  upon  local  conditions  and 
vary  greatly.  Valley  glaciers  are  long  or  short,  wide  or 
narrow,  rough  or  smooth,  according  to  the  form  of  the  val- 
ley and  the  extent  of  the  snow  fields  above  it.  Similarly 
ice  sheets  may  spread  out  for  short  distances  from  moun- 
tains, as  south  of  Mount  St.  Elias,  or  they  may  cover  vast 
areas,  as  in  Greenland.  Likewise  the  rate  of  movement  is 
variable,  though  always  slow,  and  their  geological  efficiency 
is  in  many  degrees  and  kinds.  Agassiz,  many  years  ago, 
well  observed  that  the  study  of  a  single  glacier  was  quite 
inadequate  to  the  general  understanding  of  the  subject. 
Within  the  past  decade  the  glaciers  of  Alaska  and  Green- 
land have  yielded  a  great  body  of  fresh  facts,  and  the  antarc- 
tic regions  may  prove  to  be  even  more  instructive.  Gla- 
ciers have  been  a  most  important  factor  in  the  history  of 
the  earth,  and  have  an  especial  interest  in  most  northern 
countries. 

.72.  Conditions  and  mode  of  formation.— The  conditions 
of  glacial  formation  are  three  in  number : 

(1)  Abundant  snowfall.  Such  a  region  is  found  in  the 
Alps,  where  the  winds,  laden  with  moisture  from  the  warm 
Mediterranean  region,  discharge  their  load,  or  similarly  on 
the  southern  shores  of  Alaska,  where  the  evaporation  from 
the  Pacific  Ocean  supplies  the  water,  and  the  cold  of  the 
90 


GLACIERS  91 

high  mountains  turns  it  into  snow.  Agassiz  records  574- 
feet  of  snow  as  falling  in  six  months  at  the  Grimsel,  and  6£ 
feet  in  one  night  at  St.  Gothard. 

(2)  A  sufficient  degree  of  cold  to  preserve  part  of  each 
winter's  fall  of  snow.     In  most  regions,  at  ordinary  alti- 
tudes, a  winter's  snow  disappears  early  in  the  following 
spring  or  summer.     But  if  the  summer  is  so  short  or  cool 
that  a  moderate  residue  is  annually  retained,  glaciers  must 
result.     It  thus  appears  that  intense  cold  is  not  essential 
to  the  formation  of  glaciers,  but  rather  a  suitable  ratio 
between  snowfall  and  temperature.     Greenland  is  a  great 
ice  field,  while  the  adjacent  parts  of  the  American  conti- 
nent are  without  glaciers.     The  same  contrast  holds  between 
the  glaciated   southern  slopes  and  unglaciated  northern 
slopes  of  the  Mount  St.  Elias  range.      Rank   vegetation 
sometimes  thrives  and  flowers  bloom  on  the  borders  of  gla- 
ciers.    The  student  should  at  the  outset  emphasize  this 
second  principle  of  glacier  formation. 

(3)  A  somewhat  extended  high  area  in  which  the  snows 
may  be  held  and  consolidated.     This  relates  more  especially 
to  mountain  glaciers.     Isolated  cones  and  steep  mountain 
slopes  may  bear  no  glaciers,  because  the  snows  are  readily 
dislodged  and  descend  chiefly  in  the  form  of  avalanches. 
But  a  deep  gorge  or  broad  shelf  on  the  mountain  side  may 
retain  snow  enough  to  form  a  glacier.     Especially  is  this 
true  of  high  basins  formed  by  several  adjacent  mountains. 
The  snows  descend  on  every  hand  by  creeping  movements 
or  by  avalanches,  and  form  a  common  mass,  out  of  which 
the  glacier,  or  group  of  glaciers,  takes  its  origin. 

73.  The  mode  of  formation  of  a  glacier.— It  must  be 
remembered  that  snow  is  only  ice  which  has  been  formed 
in  the  upper  air  in  small  crystals  of  various  shapes,  and 
that  snow  appears  white  because  light  is  freely  admitted 
into  innumerable  spaces  between  the  crystals.  As  the 
snows  become  massed  together  in  a  high  mountain  basin 
or  on  plains  below,  these  crystals  are  broken,  the  spaces  dis- 


92  GEOLOGY 

appear,  and  the  snow  assumes  the  form  and  color  of  solid 
ice.  This  process  is  a  gradual  one.  The  snow  at  first 
becomes  granular,  like  the  snow  of  early  spring,  and  in 
this  condition  the  French  call  it  neve  and  the  Germans 
term  it  Firn.  Even  the  blue  ice  of  the  solid  glacier  may 
be  broken  up  into  irregular  crystals  of  various  size,  to  which 
the  name  glacier  corn  has  been  given.  The  glaciers  will 
follow  the  lowest  passes  or  valleys  which  lead  out  from  the 
region  of  accumulation.  If  we  ascend  a  glacier  stream  in 
the  summer  we  shall  first  traverse  a  field  of  ice,  whose  sur- 
face is  rapidly  melting.  Farther  up  we  come  to  a  por- 
tion of  the  glacier  which  is  covered  with  the  unmelted 
snows  of  the  last  winter.  At  the  depth  of  a  few  inches  or 
a  few  feet  one  may  pierce  to  the  solid  ice.  Still  above  are 
the  steep  slopes  covered  with  the  later  snows.  These  may 
reach  to  the  summit,  or  peaks  and  ridges  on  which  no  snow 
can  lie  may  rise  out  of  the  snow  fields.  There  is  no  sharp 
line  at  which  the  glacier  ceases  and  the  snow  field  begins. 
And  the  so-called  snow  line  is  most  irregular.  The  amount 
of  snowfall,  the  steepness  of  the  slope,  and  the  relation  of 
the  slope  to  the  noonday  sun,  cause  the  line  of  perpetual 
snow  to  vary  greatly  even  in  adjoining  regions. 

THE  MOTION  OF  GLACIERS 

A  glacier  is  a  geological  agent  chiefly  by  reason  of  its 
movements.  These  will  now  claim  our  attention.  We  shall 
consider  the  proofs,  characteristics,  nature,  and  results  of 
glacier  motion. 

74.  Proofs. — The  movement  of  a  glacier  is  not  apparent 
to  the  eye.  It  appears  like  a  stagnant  mass  of  ice  strewn 
with  rocky  rubbish  and  snow.  In  1820  some  Swiss  guides 
were  lost  in  a  crevasse.  In  1860  their  remains  were  found 
at  a  distance  down  the  valley.  In  1827  a  Swiss  naturalist, 
Hugi,  built  a  hut  on  the  Unter-Aar  Glacier.  In  1841, 
according  to  Agassiz,  the  hut  had  moved  4,712  feet  from  its 


GLACIERS 


93 


f  3 


original  place.     The  movement  had  been  at  the  rate  of 

more  than  300  feet  per  year.     The  advance  of  a  glacier  has 

sometimes  destroyed  villages  in  the  valley  below.     These 

examples  may  be  called  earlier  general  proofs.     We  have 

also  the  later,  exact  measurements  of 

Tyndall  and  others.     Two  stakes  were 

planted   in   fixed   positions   on  either 

side  of  the  glacier,  and  others  between 

them,  in  a  direct  line  across  the  ice. 

Movements  of  these  relative  to  those 

fixed  at  the  ends  would  determine  the 

fact  of  motion  and  its  amount.     The 

movement  of  six  posts,  set  by  Agassiz 

across  the  Unter-Aar  Glacier,  was  as 

follows  in  one  year : 

160,  225,  269,  245,  210,  125  feet. 


FIG.  41.—  Stakes  a,  b,  c, 
etc.,  show  by  succes- 
sive positions  the  rela- 
tive movement  of  mid- 
dle and  edges  of  gla- 
cier. 


It  thus  appears  that  the  daily  mo- 
tion was  from  4  to  9  inches  per  day.  The  wide  glacial 
streams  of  Alaska  and  the  Greenland  coast  move  much 
more  rapidly  than  this.  Some  Greenland  glaciers  move 
from  40  to  60  feet  per  day,  and  even  a  higher  rate  has 
been  reported  in  a  few  cases. 

75.  Characteristics  of  glacier  motion.  —  It  has  been  found 
that  in  several  respects  the  motion  of  a  glacier  is  like  that 
of  a  river.  Thus  the  most  rapid  movement  is  in  the  middle 
portions.  The  sides  of  the  ice  stream  are  detained  by  the 
friction  of  the  valley  walls.  The  measurements  given  above 
show  this  as  well  as  the  following,  given  in  inches  per  day  : 

5,  8,  10,  9,  9,  8,  6,  9,  7,  6  inches. 

Here  we  have  near  the  middle,  a  point  of  less  motion, 
probably  due  to  obstruction  under  the  glacier.  A  glacier 
moves  more  rapidly  at  the  top  than  at  the  bottom.  The 
movement  is  also  greater  in  summer  than  in  winter,  and 
more  rapid  during  the  day  than  in  the  night.  Several 


GEOLOGY 


FIG.  42.— Stakes  a,  b,  c  show  by  successive 
positions  the  relative  movement  of  upper 
and  lower  portions  of  glacier. 


broad  glaciers  may  unite  into  a  trunk  stream,  which  is 
much  narrower  than  the  combined  width  of  the  tributary 
streams.  The  trunk  glacier  in  this  case  must  be  deeper  or 

must  move  more  rapid- 
ly. Here  the  similarity 
to  rivers  is  close,  as,  for 
example,  to  the  whirl- 
pool gorge  of  Niagara. 
The  ice  stream  behaves 
much  like  a  river  in 
passing  obstacles.  The 
ice  molds  itself  to  the 
form  of  its  channel  as 

perfectly  as  water.  The  illustration  of  the  winding  Viesch 
Glacier  (frontispiece)  shows  this.  A  rocky  hummock  in 
the  track  of  the  glacier  will  be  overridden,  if  not  too  high, 
as  the  water  of  a  torrent  rises  over  a  bowlder  in  its  bed.  If 
the  obstruction  rises  through  the  glacier,  the  ice  will  flow 
up  on  the  side  of  approach, 
and  sink  away  from  the  lee 
or  protected  face. 

76.  Nature  of  glacier  mo- 
tion.— Much  has  been  writ- 
ten, but  our  knowledge  is  not 
definite.  According  to  one  of 
the  chief  theories,  ice,  brittle 
as  it  is,  is  slightly  viscous,  and 
hence  will  flow  under  pres- 
sure. It  is  urged,  in  illustra- 
tion, that  pitch,  such  as  as- 
phalt, will  flow  with  sufficient 
time,  but  fractures  under  the 
hammer  like  any  other  brittle 

substance.  The  same  would  be  true  of  a  bar  of  molasses 
candy.  Another  theory  is  that,  under  great  pressure  above 
and  from  behind,  the  ice  is  minutely  broken,  and  the  par- 


FIG.  43.  —  Converging  and  diverging 
flow  of  glacial  ice  between  eleva- 
tions.—After  SHAI.ER. 


GLACIERS 


95 


tides  united  again  by  freezing,  after  undergoing  a  slight 
relative  change  of  position.  For  other  hypotheses  the  stu- 
dent is  referred  to  larger  works.  A  good  account  may  be 
found  in  Le  Conte's  Elements  of  Geology. 

77.  Results  of  glacier  motion. — Here  we  have  certain  ele- 
ments of  structure,  erosion,  and  transportation. 


FIG.  44.— Looking  south  across  the  Unter-Aar  Glacier.    Crevasses  and  medial 
moraine.    Photograph  by  the  author. 

78.  Elements  of  structure  due  to  motion.— Most  obvious 
of  these  are  the  crevasses  due  to  unequal  strain.  Thus  the 
glacier  moves  faster  in  the  middle  than  toward  the  sides, 
and  it  is  found  that  such  strain  forms  fissures,  running  up- 
ward and  inward  from  the  sides.  Others  are  formed  when 
a  glacier  descends  over  a  sharp  slope  in  its  bed,  as  in  the 


96 


GEOLOGY 


great  ice  fall  of  the  Khone  Glacier.  The  hummocks  and 
pinnacles  thus  formed  are  called  seracs.  There  is  also 
found  a  thin  vein  structure  or  banding  of  layers  of  white 
and  blue  ice,  believed  to  be  due  to  pressure.  The  appear- 
ance of  stratification  is  sometimes  seen,  with  dirt  bands, 
due  to  successive  deposits  of  snow  with  debris-covered  sur- 
faces ;  or  sometimes  the  layers  of  rubbish  are  carried  into 
the  heart  of  the  glacier  from  rocky  obstructions,  which  are 
steadily  eroded  away. 

79.  Erosion. — The  thickness  of  a  glacier  may  be  several 
hundred  feet,  or,  in  the  case  of  ice  sheets,  several  thousand 
feet.  The  movement  of  so  heavy  a  mass  of  solid  matter 
exerts  a  powerful  rending  force  upon  the  rocks  over  which 
it  passes.  Materials  whose  cohesion  has  been  lessened  by 
weathering,  and  blocks  due  to  bedding,  joints,  and  cleavage 


Fio.  45.— Glacial  scorings.— After  AGASS 


are  readily  plucked  from  the  bottom  and  sides  of  a  valley. 
The  lower  parts  of  the  ice  stream  are  set  with  such  masses, 
which  serve  as  graving  tools,  and  are  rasped  with  great 


GLACIERS  97 

energy  over  the  bed  rocks  beneath.  By  this  means  not 
only  are  rocks  dislodged  from  below,  but  large  quantities  of 
fine  rock  flour  are  produced.  The  under  rocks  are  scored 
and  graven  in  a  most  characteristic  manner.  Such  marks 
are  known  as  glacial  striae,  and  such  a  surface  is  often 
said  to  be  glaciated.  The  gravings  may  be  fine,  as  if  done 
by  a  delicate  point,  or  coarse  and  rough,  as  when  made  by 
the  pushing  of  a  heavy  and  angular  bowlder.  Sometimes 
quite  elaborate  flutings  and  moldings  are  made  in  this 
manner.  They  are  often  to  be  found  where  soil  and  drift 
are  stripped  from  the  rock,  as  for  quarrying.  The  more 
delicate  markings  are  best  taken  by  fine-grained  rocks,  such 
as  many  limestones.  The  striae  are  useful,  not  only  as 
proving  glacial  action,  but  because  in  general  they  show 
the  direction  of  the  ice  movements.  Rocky  eminences 
rising  in  the  track  of  the  ice  are  smoothed  down  and 
carved  into  low,  elongated  ridges  and  swells,  known  as 
roclies  moutonnees,  from  their  resemblance  to  the  backs  of 
a  flock  of  sheep.  It  is  evident  that  the  stones  pushed 
under  the  ice  must  be  similarly  scratched,  and  hence  we 
find  the  glaciated  pebble,  a  most  characteristic  product.* 

80.  Transportation  by  glaciers. — Glaciers  which  occupy 
valleys  with  steep  sides,  especially  among  high  mountains, 
receive  large  contributions  of  rocky  and  earthy  debris  from 
above  by  the  ordinary  processes  of  weathering,  by  ava- 
lanches and  landslips.  It  is  evident  that  this  material  will 
share  the  motion  of  the  glacier.  Some  of  these  fragments 
from  time  to  time  descend  into  the  ice  by  means  of  cre- 
vasses and  glacier  mills.  Other  masses  are  carried  into  the 
heart  of  the  ice  from  rocky  heights  that  rise  into  the  gla- 
cier, or  by  means  of  cross  currents  within  the  ice.  Much 
rock  is  also  taken  into  the  lower  portions  of  the  ice,  as 
already  described.  A  glacier  thus  transports  materials 
upon,  within,  and  under  its  mass.  Such  material  is  tech- 
nically called  superglacial,  englacial  or  intraglacial,  and 
subglacial.  Great  ice  sheets  which  cover  the  high  grounds 

*  See  Fig.  285. 


98 


GEOLOGY 


of  a  region  carry  most  of  their  rocky  burden  at  or  near  the 
bottom.  An  interesting  difference  between  transportation 
by  a  river  and  carriage  by  glaciers  lies  in  the  fact  that  the 
size  of  the  stones  carried  by  the  glacier  bears  no  relation  to 
its  velocity.  The  slow  rate  of  travel  should  not  be  thought 
to  destroy  .the  importance  of  glacial  transportation.  The 
student  should  ever  seek  to  enlarge  his  conception  of  the 
time  which  is  available  for  the  earth's  history.  There  is 
abundant  proof  of  the  carriage  of  material  for  several  hun- 
dred miles  in  Xorth  America  and  northern  Europe.  A 
fuller  account  of  these  facts  belongs  to  the  section  on  His- 
torical Geology. 

81.  Moraines. — A  moraine  is  an  accumulation  of  rocky 
matter  carried  by  the  glacier,  or  deposited  by  it  through 


FIG.  46. -Looking  np  the  left  lateral  moraine  of  the  Tschingel  Glacier, 
Switzerland.    Photograph  by  the  author. 

melting.  It  might  therefore  with  some  reason  be  treated  in 
the  next  section.  But,  as  will  be  seen,  certain  kinds  of  mo- 
raines take  form  on  the  moving  glacier.  On  the  borders  of 


GLACIERS  99 

a  valley  glacier  stony  rubbish  gathers,  both  by  plucking  and 
by  descent  from  cliffs.  This  material  forms  lateral  mo- 
raines. It  is  in  part  borne  by  the  glacier  and  in  part 
banked  against  the  valley  sides.  When  the  glacier  melts 
away,  the  lateral  moraine  is  left  as  a  narrow  ridge,  often  of 
sharp  crest  and  steep  slopes,  straight  or  curved,  according 
to  the  form  of  the  valley. 

When  two  glaciers  unite  in  a  trunk  stream,  the  adjacent 
lateral  moraines  form  a  medial  moraine,  which  is  carried 
on  the  surface  as  a  ridge  of  angular  bowlders.  Thus  the 
medial  moraine  of  the  Unter-Aar  Glacier  is  several  miles 
long,  often  100  feet  high,  and  several  hundred  feet  wide. 
If  several  glaciers  unite  into  one  stream,  a  number  of  me- 
dial moraines  will  appear,  parallel  to  each  other. 

The  most  important  and  complex  accumulations  are 
made  at  the  front  or  terminus  of  the  glacier,  and  are  called 
terminal  moraines.  Materials  carried  under  or  within  the 
glacier  must  come  to  rest  where  the  ice  disappears  by  melt- 
ing. In  like  manner  materials  which  travel  to  the  front  as 
part  of  a  lateral  or  medial  moraine  mingle  with  the  ter- 
minal mass.  Large  frontal  areas  of  the  glacier  may  be  hid- 
den by  such  waste.  When  the  glacier  recedes  by  melting, 
the  terminal  moraine  may  appear  as  a  crescentic  ridge 
with  its  concave  side  up  the  valley,  or  it  may  be  a  hum- 
mocky  and  irregular  assemblage  of  material  occupying  the 
valley  for  some  distance.  The  latter  is  the  case  if  the  posi- 
tion of  the  front  is  subject  to  considerable  oscillation. 
Especially  in  front  of  the  ancient  ice  sheets  are  the  mo- 
raines found  to  be  broad  and  intricate  belts  of  knolls  and 
ridges,  rather  than  narrow  and  well-defined  lines  of  accu- 
mulation. A  succession  of  pauses  in  the  melting  away  of 
a  glacier  may  give  a  succession  of  terminal  moraines. 

Wherever  a  glacier  has  disappeared  by  melting,  the 
area  is  usually  occupied  by  a  sheet  of  rocky  and  earthy  mat- 
ter, known  as  the  ground  moraine.  This  has  its  chief  im- 
portance in  connection  with  the  ancient  ice  sheets. 


FIG.  47.— Alpine  glacier ;  crevasses  uud  formation  of  medial  moraine. 


GLACIERS 


101 


Material  of  moraines. — The  lateral  and  medial  moraines 
consist  chiefly  of  angular,  unworn  bowlders  and  smaller 
fragments.  Terminal  moraines,  on  the  other  hand,  contain 
many  rounded  and  worn  masses,  abraded  under  the  ice  or 


FIG.  48.— Pool  on  glacier.    Ice  thinly  covered  with  angular  bowlders. 
Photograph  by  the  author. 

rolled  in  glacial  waters.  Much,  and  sometimes  all,  of  a 
terminal  moraine  may  consist  of  tough,  clayey  matter  or 
rock  flour,  more  or  less  set  with  stones.*  Much  water-worn 
and  stratified  matter,  however,  often  belongs  also  to  these 
terminal  accumulations.  These  differences  depend  upon  a 
great  variety  of  special  conditions. 

THE  MELTING  OF  GLACIERS 

82.  Whenever  the  season  permits  melting  takes  place 
everywhere  on  the  surface  of  a  glacier,  and  to  some  extent 


8 


*  Bowlder  clay,  or  till.    See  p.  428. 


102  GEOLOGY 

within  its  mass  and  at  its  base.  In  the  last  case  something 
is  due  to  internal  and  basal  friction.  A  number  of  char- 
acteristic phenomena  thus  arise.  Rills,  and  sometimes 
small  torrents,  rush  down  the  surface,  usually  to  disappear 
in  crevasses  or  well-like  openings  in  the  ice.  This  water 
reaches  the.  bottom  and  forms  the  subglacial  stream  which 
issues  from  all  glaciers,  usually  by  a  broad,  low  tunnel  or 
arch.  Thus  the  Aar  River  issues  from  the  Unter-Aar  Gla- 


Fio.  49.-Anr  River  flowing  from  beneatl 
Photograph  by  the  author. 


cier,  and  the  Rhone  River  from  the  Rhone  Glacier.  The 
subglacial  stream,  like  an  open-air  torrent,  is  a  powerful 
instrument  of  erosion.  Its  carrying  work  is  even  more 


GLACIERS 


103 


conspicuous.  Materials  gathered  under  the  ice  or  picked 
from  the  terminal  moraine  are  spread  out  in  stony  alluvial 
areas  in  the  valley  below.  Especially  does  the  subglacial 
stream  carry  much  rock  flour,  the  product  of  glacial  abra- 
sion. So  whitened  by  this  means  are  some  streams  that 
their  waters  are  called  glacier  milk.  Two  hundred  and 
eighty  tons  of  sand  were  found  to  have  been  discharged 
in  one  August  day  by  the  stream  flowing  from  the  Aar 
Glacier. 

83.  Honeycombed  surface  and  glacier  tables. — During 
summer  days  melting  is  very  active,  and  the  surface  of  a 
glacier  may  consist  of  innumerable  small  pinnacles  and 
pits.  Small  pebbles  lying  on  the  surface  absorb  the  heat, 
quicken  the  melting,  and  thus  sink  into  the  ice.  On  the 
other  hand,  large  slabs  or  bowlders  protect  the  ice  beneath 


Fio.  50.— Glacier  table.    Ice  pedestal  melting  on  e 

Photograph  by  the  author. 


ide,  Unter-Aar  Glacier. 


from  the  heat,  while  the  adjacent  ice  melts  away,  leaving 
the  slabs  mounted  upon  ice  pedestals  sometimes  several  feet 
in  height.  The  south  side  of  the  pedestal  will  melt  most 


104 


GEOLOGY 


rapidly,  and  the  slab  will  incline  in  that  direction,  until  at 
length  it  slides  off  and  the  process  is  repeated  in  its  new 
position.  One  slab  seen  by  the  writer  was  elevated  on  two 
ice  pillars,  between  which  flowed  a  vigorous  stream  of  water. 
The  stream  had  found  its  way  under  the  bowlder  while  the 
stone  lay  on  the  common  level,  and  had  sunk  its  channel 
while  the  sun  lowered  the  surrounding  ice. 

84.  Ice  pinnacles. — A  layer  of  sediment  may  gather  in  a 
pool  on  the  glacier.  The  underlying  ice  is  thus  protected 
from  the  heat  while  the  surrounding  ice  melts  away.  The 


FIG.  51.— Ice  pinnacles  veneered  with  waste,  Unter-Aur  Glacier. 
Photograph  by  the  author. 

protected  ice  assumes  at  length  the  form  of  a  sharp  cone, 
covered  with  a  very  thin  layer  of  sand.  In  a  cone  several 
feet  high  and  of  sharp  apex  the  ice  may  rise  to  within  an 
inch  of  the  top.  It  will  be  seen  that  the  ice  pinnacle  is 
closely  related  to  the  glacier  table. 

85.  Glacier  mills. — A  surface  stream  often  discharges  into 
a  crevasse,  causing  it  to  widen  at  that  point.  If  now  the 
crevasse  closes,  a  round  or  oval  opening  is  left,  receiving 
surface  waters,  which  in  turn  may  erode  or  melt  the  ice  to 
form  irregular  or  spiral  pits  descending  profoundly  into 
the  glacier.  Thus  we  have  the  glacier  mill.  A  change  in 


GLACIERS 


105 


the  stream  may  leave  the  pit  dry.  Agassiz  descended  to  a 
depth  of  125  feet  in  one  of  these,  to  study  the  interior 
structure  of  the  ice,  whose  banding  and  color  could  thus 
be  seen  to  advantage. 


FIG.  52.— Abandoned  glacier  mi! 
Photograph  by  the 


Unter-Aar  Glacier, 
athor. 


EXISTING  GLACIERS 

A  full  account  of  existing  glaciers  belongs  to  physical 
geography.  We  here  review  the  subject  briefly,  giving  the 
distribution  of  the  great  types  of  glacier.  This  will  also  aid 
in  understanding  ancient  glaciation. 

80.  Valley  glaciers. — These  are  well  developed  in  Switz- 
erland, where  they  are  found  to  the  number  of  many  hun- 
dreds, are  most  often  visited,  and  have  for  many  years  been 
carefully  studied.  The  chief  glacier  regions  of  Switzerland 
are  three:  The  vicinity  of  Mo?t  Blanc,  with  the  Mer  de 


10g  GEOLOGY 

Glace  and  other  ice  streams ;  about  Zermatt,  where  several 
large  glaciers  stream  down  from  the  Monte  Rosa  chain  to 
form  the  huge  Corner  Glacier ;  and  the  Bernese  Oberland. 
In  the  last  we  have  among  many  others  the  Aletsch,  the 
largest  ice  stream  in  Switzerland ;  the  Grindelwald,  reaching 
down  within  3,000  feet  of  the  sea  level ;  and  the  Unter-Aar, 
made  famous  by  the  studies  of  Agassiz  and  others.  There 
are  many  glaciers  in  the  eastern  Alps,  in  the  Pyrenees  and 
Caucasus,  and  in  the  mountain  valleys  of  Norway.  Con- 
siderable glaciers  are  found  in  the  Himalayas,  but  of  these 
little  is  known.  Small  valley  glaciers  occur  in  the  high 
mountains  of  California,  Oregon,  and  Washington,  larger 
ones  in  the  mountains  of  British  America,  and  some  of 
great  size  in  southern  Alaska,  among  them  the  celebrated 
Muir  Glacier. 

87.  Piedmont  (foot  of  the  mountain)  glaciers. — A  single 
glacier  of  this  type  has   been  described  by  Eussell,  the 
Malaspina,  extending  from  the  foot  of  the  Mount  St.  Elias 
range   to   the   sea,  30  miles,  and   having  a  width  of   70 
miles.     It  is  formed  by  several  valley  glaciers  merging  on 
the  plain,  and  its  stagnant  border  on  the  south  is  covered 
along  a  belt  several  miles  wide  with    morainic   soil  and 
extensive  forests.     (See  Fig.  54.) 

88.  Ice  sheets. — These  are  of  special  geological  interest, 
because  they  illustrate  at  the  present  day  the  conditions 
and  size  of  the  ancient  ice  fields.    The  best  known  of  these 
sheets  is  in  Greenland,  whose  entire  interior  to  the  extent 
of  several  hundred  thousand  square  miles  is  mantled  with 
ice,  covering  all  elevations.     It  is   comparatively  smooth, 
and  carries  on  its  surface  but  a  slight  amount  of  rocky 
debris.     From  this  ice  field  tongues  or  streams  flow  down 
the  valleys  which  lead  from  the  interior  to  the  sea.     The 
interior  ice  has  been  explored  by  Xordenskiold,  Xansen, 
and  Peary,  while  the  ice  streams  that  enter  the  sea  have 
been  seen  by  many  arctic  explorers,  and  in  recent  years 
have  been  studied  by  Chamberlin  and  others.     The  Hum- 


108 


GEOLOGY 


boldt  Glacier  enters  the  sea  with  a  width  of  60  miles.  A 
vast  ice  sheet,  perhaps  equal  in  extent  to  any  that  existed 
in  the  Ice  age,  covers  the  antarctic  region.  Little  is 


PIG.  54. — Vegetation  on  moraine-covered  portion  of  the  Malaspina  Glacier, 
four  miles  fron,  the  front  of  the  ice. 


known  of  it,  with  exception  of  its  steep  seaward  cliffs, 
along  which  vessels  have  sailed  for  several  hundred  miles. 
It  offers  a  great  field  for  glacial  study. 

89.  Icebergs.— The  mountain-like  masses  of  ice  which 
float  in  the  Atlantic,  and  to  some  extent  in  other  marine 
waters,  have  their  origin  in  glacial  streams.  The  front  of 
the  glacier  is  buoyed  up  as  it  enters  water  which  is  deeper 


GLACIERS  109 

than  its  own  thickness,  and  huge  masses  break  loose  and 
float  away,  bearing  the  waste  which  may  rest  on  their  sur- 
faces or  be  frozen  within  their  mass.  They  may  be  carried 
to  distant  latitudes  by  ocean  currents  before  they  disappear 
by  melting  and  contribute  their  deposits  to  the  sea  bottom. 
Where  the  cliffs  of  a  glacier  front  rise  high  above  the  mar- 
ginal waters  of  a  sea  or  lake  small  icebergs  form  by  the 
"  calving "  off  of  crevassed  masses,  which  fall  into  the 
water  and  float  away.  Thus  Glacier  Bay  is  often  covered 
with  small  bergs  from  the  front  of  the  Muir  Glacier.  Ice- 
bergs and  their  deposits  were  of  some  importance  in  the 
Glacial  period  in  the  region  of  the  Great  Lakes. 

90.  Avalanches. — In  all  regions  of  high  mountains  ava- 
lanches have  geological  importance.  On  steep  slopes  above 
the  snow  line  masses  of  old  and  new  snow  are  often  dis- 
lodged to  descend  with  terrific  force  into  the  valleys. 
Trees  lying  in  their  path  are  destroyed,  and  avalanche 
tracks  may  often  be  seen  running  down  the  forest  slopes 
in  parallel  or  radiating  lines.  Well-defined  avalanche 
tracks  are  found  in  the  Eocky  Mountains,  as  on  the  slopes 
about  Silverton,  Col.  Avalanches  sometimes  occur  among 
the  WThite  Mountains  of  New  Hampshire.  So  numerous 
are  the  avalanche  paths  in  the  Alps  that  an  elaborate  offi- 
cial map  of  them  has  been  prepared  for  publication.  These 
tracks  may  readily  be  adopted  by  mountain  torrents,  and 
erosion  thus  be  carried  on  indefinitely.  Avalanche  snows 
lying  on  the  lower  slopes  are  often  thickly  set  with  stones 
and  rough  bowlders  brought  from  above,  and  the  destruc- 
tion of  life  and  property  by  them  is  sometimes  serious. 
Even  the  wind  generated  by  the  movement  of  a  great  ava- 
lanche may  suffice  to  prostrate  a  forest.  Similarly  ice  falls 
occur.  From  the  so-called  hanging  glaciers,  perched  on 
lofty  mountain  shelves,  masses  crack  off,  forming  a  dense 
cloud  of  pulverized  ice  as  they  are  crushed  by  their  fall,  or 
pouring  as  a  torrent  through  narrow  ravines  in  the  lower 
mountain  slopes. 


CHAPTER  VI 

LAKES 

91.  Definition. — A  lake  is  a  body  of  surface  waters,  lying 
apart  from  the  sea,  usually  above  it,  and  detained  by  a  natu- 
ral barrier.     Lakes  occur  wherever  inclosed  basins  have  in 
any  manner  been  formed,  if  the  bottom  is  sufficiently  im- 
pervious, and  if  there  be  an  adequate  supply  of  water. 
They  differ  from  the  ocean  in  size,  in  the  position  of  their 
surface,  which  is  usually  above,  though  rarely  below,  sea 
level ;  and  in  the  character  of  their  waters,  which  are  com- 
monly fresh,  but  in  a  few  cases  are  more  salt  than  the  sea. 
They  sustain  a  close  relation  to  rivers,  of  which  they  may 
be  considered  as  lobelike  expansions.     Thus  the  St.  Law- 
rence River  is  more  properly  regarded  as  having  its  head 
waters  in  Minnesota  and  Canada,  and  as  passing  through 
the  chain  of  the  Great  Lakes  on  its  way  to  the  sea.     Lakes 
are  comparatively  short-lived  features  of  the  earth's  sur- 
face.    Many  land  surfaces  have,  however,  been  formed  and 
destroyed  or  profoundly  modified  during  the  earth's  history, 
and  hence  there  must  have  been  many  generations  of  lakes. 
The  basins  are  physiographic  forms,  and  will  therefore  be 
more  fully  treated  under  that  head  (see  Chapter  XIV).   This 
is  the  more  necessary  because  they  are  often  of  highly  com- 
posite origin,  a  variety  of  geological  forces  and  structures 
combining  to  make  them  what  they  are.     We  are  here  only 
concerned  with  the  geological  work  which  lakes  may  be  said 
to  perform  and  for  which  they  furnish  the  opportunity. 

92.  Geological  work  of  lakes.— The  chief  movements  of 
lake  waters  are  waves  produced  by  the  winds.     By  their 

110 


' 


112  GEOLOGY 

means  erosion  is  accomplished  upon  shores  and  on  shallow 
bottoms.  In  the  case  of  small  lakes  the  amount  of  such 
work  is  slight,  but  on  large  lakes  the  winds  sweep  power- 
fully, waves  roll  high,  shore  cliffs  are  formed,  and  much 
rock  is  broken  up  and  distributed  over  the  lake  bottom. 
This  work. is  similar  to  that  of  the  ocean,  and  the  student 
is  referred  to  that  subject  for  fuller  treatment.  Lake  bot- 
toms, like  sea  bottoms,  are  everywhere  the  recipients  of 
fragmental  materials  brought  from  adjacent  lands.  Xear 
the  shore  coarser  fragments  worn  by  waves  or  brought  by 
streams  are  laid  down.  At  the  mouth  of  streams  deltas  are 
formed,  precisely  as  in  tideless  seas,  with  the  difference  that 
streams  of  swift  flow  often  enter  lakes  directly,  their  load 
is  dropped  suddenly,  and  the  deltas  afford  highly  inclined 
beds  and  frontal  slopes.  Beneath  the  deeper  offshore  waters 
of  lakes,  as  well  as  seas,  the  finer  sediments  come  to  rest,  and 
both  shore-  and  deep-water  deposits  may  contain  the  forms 
of  plants  and  animals  which  once  inhabited  the  waters,  or 
whose  remains  have  been  drifted  in  by  streams.  In  large 
lakes  currents  of  importance  may  be  generated  by  winds, 
and  may  be  an  efficient  means  of  transportation,  particu- 
larly for  the  redistribution  of  sediments  alongshore.  Lake- 
bottom  deposits  have  afforded  much  instruction,  because 
in  some  cases  lakes  have  disappeared,  leaving  their  varied 
shore  lines,  deltas,  and  bottoms  perfectly  exposed  for  study. 
Thus  we  have  the  Lakes  Bonneville  and  Lahontan,  and 
the  glacial  extensions  of  the  Laurentian  lakes  (see  Chap- 
ter XXV). 

Lakes  are  also  economically  important,  because  they  reg- 
ulate the  flow  of  river  waters,  serving  as  reservoirs  by  which 
the  water  that  falls  on  the  higher  grounds  is  detained  and 
sent  on  gradually  to  the  sea,  thus  averting  floods  and  modi- 
fying the  conditions  of  vegetable  life.  They  also  modify 
the  climate  of  surrounding  regions  by  delaying  or  averting 
frosts,  and  by  offering  surfaces  over  which  large  evapora- 
tion and  cloud  formation  take  place. 


114 


GEOLOGY 


93.  Salt  lakes. — We  have  already  seen  that  water  which 
flows  over,  or  within  the  rocks  and  soil,  carries  some  min- 
eral matter  in  solution.  In  the  ordinary  lake  with  con- 
stant inflow  and  outflow,  this  percentage  of  solid  material 
does  not  increase.  But  if  conditions  arise  in  which  there 
is  no  outflow  either  on  or  under  the  surface  of  the  earth, 
mineral  matter  will  continue  to  be  brought  in  by  streams, 
but  only  pure  water  can  escape  by  evaporation.  Hence 
mineral  salts,  common  salt,  and  others,  must  accumulate 
in  the  lake.  This  process  may  go  on  until  the  water  is 
saturated  and  deposition  of  the  surplus  salts  takes  place, 
as  in  Great  Salt  Lake,  the  Dead  Sea,  and  similar  bodies  of 
water.  A  salt  lake  can  only  exist  when  the  size  of  the 
drainage  basin  and  the  amount  of  evaporation  going  on 
in  the  region  is  large,  relative  to  the  rainfall.  Thus  the 
basin  of  Great  Salt  Lake  is  large,  has  little  rainfall,  and  a 
very  dry  atmosphere.  Hence,  although  there  is  a  consid- 
erable flow  of  water  from  the  Wasatch  Mountains  on  the 
east,  the  basin  can  not  fill  up  and  overflow  its  rim.  But  in 
earlier  times  the  climate  was  moist,  and  a  lake  of  the  size 
of  Lake  Huron  existed,  with  outflow  to  the  north.  Thus  a 
change  of  climate  may  cause  a  fresh-water  lake  to  become 
salt,  or  a  salt  lake  to  become  fresh.  Similar  results  may 
arise  from  movements  of  the  land.  Thus  the  valley  of 
Lake  Champlain  was  once  occupied  by  marine  waters.  By 
an  uplift  of  eastern  North  America  this  valley  was  cut  off 
from  the  inflow  of  the  sea,  but  abundant  rains  kept  the 
basin  full  and  overflowing,  and  the  salty  waters  were  at 
length  replaced  by  fresh.  The  Caspian  Sea,  on  the  other 
hand,  retains  its  salt  waters  after  being  severed  from  the 
ocean.  This  is  because  it  occupies  a  great  basin,  over  most 
of  which  rapid  evaporation  always  takes  place,  and  even  the 
Volga  can  not  supply  sufficient  waters  to  fill  up  the  basin 
and  produce  an  overflow.  Some  shallow  lakes  in  dry  re- 
gions, as  Nevada,  receive  deposits  of  mud  over  their  bot- 
toms during  the  brief  period  of  rains.  The  waters  then 


LAKES  115 

dry  away,  leaving  an  incrustation  of  salts  over  the  mud. 
The  same  thing  takes  place  many  times,  affording  alter- 
nations of  shaly  rock  and  of  salt.  We  shall  have  further 
occasion  to  refer  to  this  process  in  reviewing  the  origin  of 
the  great  deposits  of  rock  salt. 


CHAPTER  VII 

THE  OCEAN 

94.  THE  ocean  is  indirectly  the  source  of  the  geological 
efficiency  of  water  in  all  its  forms.     From  it  all  land  waters 
come,  and  to  this  common  reservoir  they  all  return.     The 
work  of  the  sea  is  partly  destructive,  but  to  a  higher  degree 
it  is  constructive.     The  greater  bodies  of  sedimentary  rock 
have  all  been  formed  upon  the  ocean's  bed,  and  there  also 
our  most  connected  and  full  record  of  the  ancient  life  of 
the  globe  has  been  made.     On  the  whole,  such  records  are 
destroyed,  rather  than  made,  over  the  lands.     But  in  the 
sea  successive  generations  of  creatures  have  lived  and  died, 
depositing  their  remains  in  a  growing  series  of  rocks.    This 
phase  of  the  ocean's  work  will  receive  constant  illustration 
in  the  treatment  of  historical  geology,  and  hence  need  not 
be  further  considered  here. 

95.  Movements  of  ocean  waters. — Marine  waters  are  geo- 
logical agents,  particularly  of  erosion  and  transportation, 
by  reason  of  their  movements.     These  movements  fall  into 
three  classes— viz.,  tides,  currents,  and  waves. 

(1)  Tides.— On  the  seashore  the  water  is  seen  to  rise 
and  fall  twice  in  a  little  more  than  twenty-four  hours.  The 
range  of  oscillation  varies  from  a  few  feet  on  more  open 
shores  to  more  than  50  feet  in  such  inlets  as  the  Bay  of 
Fundy  and  the  estuary  of  the  Severn.  The  tides  are  due 
chiefly  to  the  attraction  of  the  moon,  exerted  both  upon 
the  mass  of  the  earth  and  upon  its  less  stable  mantle  of 
water.  In  the  open  sea  the  tidal  wave  is  broad  and  imper- 
116 


THE  OCEAN 


117 


Ceptible,  but  manifests  itself  on  the  shore.  In  estuaries, 
where  the  wave  is  concentrated  and  may  rise  to  a  great 
height,  a  rapid  inflow  and  outflow  take  place,  by  which  ero- 
sion and  much  transportation  are  effected.  On  open  shores 
the  rise  of  the  tides  is  so  gentle  that  they  have  little  force ; 


FIG.  57.— Mud  flat  at  low  tide,  Minas  Basin,  Nova  Scotia. 

(Copyright,  1888,  by  S.  R.  STODDARD,  Glens  Falls,  X.  Y.) 

but  they  are  important  as  a  means  of  deposit  along  low 
shores  where  tidal  flats  are  growing.  The  loss  of  velocity 
causes  deposit  as  the  tide  recedes,  little  movement  being 
generated  by  the  shallow  outflowing  waters,  and  the  muds 
being  entrapped  by  grasses  and  other  plants.  This  prin- 
ciple is  used  in  reclaiming  areas  from  the  sea  border.  An 


118  GEOLOGY 

embankment  is  built  about  the  area,  which  is  easily  covered 
at  high  tide.  Sediments  gather  in  the  still  water  behind 
the  barrier,  and  the  outflowing  tide  is  powerless  to  remove 
them.  Gradually  the  surface  rises  above  the  tide,  or  to  a 
plane  at  which  a  slight  embankment  will  protect  it  from 
further  overflow. 

(2)  Currents. — These  are  broad  and  massive  flows  of  ocean 
waters.     They  are  due  chiefly  to  prevailing  winds,  which  in 
turn  depend  on  the  earth's  rotation  and  the  difference  in 
temperature  between  the  equatorial  regions  and  those  of 
high  latitude.    The  direction  of  the  currents  is  largely  con- 
trolled by  the  arrangement  of  the  lands.     In  the  Atlantic 
Ocean  a  great  current  moves  westward  along  the  equator. 
Part  of  this  current  passes  northward  through  the  Carib- 
bean Sea  and  the  Gulf  of  Mexico,  then  northeastward  past 
Newfoundland  to  the  eastern  shore  of  Europe,  and  returns 
in  part  southward  along  the  West  African  coast,  completing 
the  circuit.     In  like  manner  the  rest  of  the  current  moves 
south,  east,  and  north  in  the  south  Atlantic.     Currents  sim- 
ilar in  direction  occupy  the  north  and  south  Pacific.     One 
test  of  the  existence  and  direction  of  ocean  currents  is  in 
the  transport  of  objects  of  known  origin.     A  cask  of  palm 
oil  set  adrift  off  West  Africa  came  ashore  at  Hammerfest, 
Norway,  in  one  year.     It  had  twice  crossed  the  Atlantic. 
West  Indian  seeds  in  like  manner  make  their  appearance 
in  Norway.     But  by  far  the  most  important  function  of 
ocean  currents  is  the  transfer  of  heat,  by  which  climates  in 
high  latitudes  are  ameliorated,  precipitation  increased,  and 
a  train  of  aqueous  and  organic  agencies  set  in  motion.    The 
most  important  illustration  of  this  principle  is  in  the  chang- 
ing of  the  entire  character  of  western  Europe,  due  to  the 
presence  of  the  Gulf  Stream  "  drift "  upon  its  shores. 

(3)  Waves. — We  here  refer  to  the  ordinary  movements 
of  water  produced  by  winds  over  more  or  less  limited  areas. 
They  become  effective  on  the  shore.     In  the  open  sea,  wave- 
stirred  waters  oscillate  through  a  curve  of  narrow  limits, 


THE  OCEAN  119 

while  only  the  transmitted  energy  passes  on.  But  in  the 
shallow  waters  of  the  sea  margin  the  wave  of  oscillation 
passes  into  a  wave  of  translation,  and  for  a  short  distance 
the  water  itself  may  move  with  great  speed  and  power. 
This  is  the  principal  means  by  which  the  sea  works  out 
varied  results  upon  its  shore.  Wave  energy  which  affects 
the  open  waters  to  a  depth  of  many  feet  is  confined  to  a 
slight  depth  near  shore.  The  water  rises,  rolls  over  on  tha 
wave's  crest,  the  wave  breaks  and  sends  its  waters  up  the 
shelving  beach,  or  dashes  them  upon  the  shore  cliffs  if 
present.  Thus  there  is  an  onshore  and  offshore  carriage 
of  coarse  or  fine  materials,  according  to  the  strength  of  the 
waves  and  the  sources  of  supply. 

There  is  also  erosion,  either  by  the  direct  blows  of  the 
water  or  of  stones  wielded  as  tools  by  the  water.  The  di- 
rect blows  of  the  water  are  in  some  cases  very  powerful.  A 
well-known  determination  was  made  on  the  coast  of  Scot- 
land. During  the  summer  months  it  was  found  that  the 
waves  dealt  a  blow  of  611  pounds  upon  a  square  foot  of  sur- 
face. During  the  winter  months  the  force  of  the  blow  rose 
to  2,086  pounds,  and  in  a  March  gale  in  1845  the  energy 
exerted  upon  a  square  foot  was  6,063  pounds.  Such  waves 
drive  air  into  crevices  in  the  rock,  and  so  compress  it  that 
it  expands  vigorously  and  thus  rends  the  inclosing  rocks. 
It  is  well  here  to  compare  the  handling  of  their  weapons 
by  river,  glacier,  and  sea.  The  weapons  are  the  same ;  the 
river  drags  them  rapidly,  the  glacier  pushes  them  slowly, 
the  sea  hurls  them  vigorously  upon  the  object  of  attack. 

96.  Erosion  on  seashores. — On  most  shores  the  work  of 
destruction  is  constant. 

"  I  with  my  hammer  pounding  evermore 
The  rocky  coast,  smite  Andes  into  dust, 
Strewing  my  bed."  EMERSON. 

The  most  effective  work  is  done  between  high  and  low  tide. 
But  the  vertical  range  of  marine  erosion  is  much  larger 
than  this.  In  great  storms  wave  action  goes  deeper  and 


THE  OCEAN 


121 


rises  higher,  and  the  sea  erodes  powerfully,  like  rivers  in 
time  of  flood.  A  door  was  wrenched  from  the  Shetland 
lighthouse  by  the  waves  at  a  height  of  196  feet  above  the 
sea  level.  In  the  same  region  blocks  of  rock  weighing  from  6 
to  13£  tons  were  plucked  from  the  parent  ledge  at  a  height 
of  70  feet  above  high  water.  Geikie  states  the  extreme 
range  of  wave  action,  below  and  above  the  mean  tide  level, 
as  300  feet.  But  the  cases  above  cited  are  exceptional,  and 
most  work  of  the  sea  on  its  border  is  accomplished  within 
a  much  smaller  vertical  interval.  Indeed,  we  may  look 
upon  the  sea  as  a  vast  saw,  operating  horizontally  upon 
the  lands.  We  take  now  the  special  phases  of  this  work. 

97.  Formation  of  sea  cliffs. — If  land  of  some  height  passes 
under  the  sea  by  an  inclined  surface,  the  sea  will  cut  a 


FIG.  59.— Sea  cliff,  Xahant,  Mass.  ;  seaweed  covering  ;  high-tide  line 
of  inclined  planes  in  the  rocks. 


122 


GEOLOGY 


notch  in  the  slope,  forming  a  cliff  landward  with  a  gently 
descending  floor  at  its  base.     The  waves  attack  the  foot  of 


FIG.  GO.— Dike  eroded  by  the  waves,  Spouting  Horn,  Marblehead  Neck,  Muss. 

the  cliff  at  high  tide  or  in  storms,  while  at  low  tide  the 
floor  is  partly  exposed,  and  is  always  strewn  with  fragments 
derived  from  the  cliffs.  These  fragments  are  seized  by  the 


THE  OCEAN  123 

waves  and  dashed  upon  the  cliffs  for  their  further  destruc- 
tion. The  waves  are  aided  by  frosts  and  by  solution,  and 
as  the  cliff  is  undermined  the  upper  masses  break  off,  and 
thus  the  cliff  recedes  landward.  The  character  of  the 
cliff  depends  upon  the  constitution  and  structure  of  the 
rock.  If  it  be  of  sand  or  clay,  the  cliff  may  have  a  mod- 
erate or  steep  inclination  toward  the  sea.  If  the  rock  is 
compact  the  cliff  may  be  vertical,  or  may  even  overhang 
the  sea  if  the  bedding,  or  joint,  or  cleavage  planes  favor  it. 
On  some  shores  no  cliffs  form,  because  they  are  regions  of 
accumulation,  and  there  is  constant  deposit  either  of  ma- 
terials brought  directly  from  the  land  or  swept  thither  by 
shore  currents.  The  cutting  of  a  cliff  is  not  uniform  at  all 
points.  If  the  rocks  at  a  given  point  be  soft,  or  peculiarly 
exposed  to  wave  action,  they  will  be  cut  away  more  rapidly, 
and  thus  sea  caverns  are  formed.  In  a  few  cases  these 
extend  inward  for  some  scores  of  feet,  as  the  celebrated 
cave  of  Staffa,  which  is  cut  for  200  feet  into  the  columns  of 
volcanic  rock  that  form  the  island.  Sometimes  the  mate- 
rial of  a  dike  is  softer  than  the  adjacent  rocks.  The  waves 
cut  away  the  dike,  driving  their  waters  with  fierce  roar 
farther  and  farther  into  the  narrow  channel  thus  exca- 
vated. At  the  inner  end  the  water  may  be  forcibly  hurled 
many  feet  into  the  air.  This  is  seen  on  the  shore  of  Mar- 
blehead  Neck. 

98.  Destruction  of  land  by  the  sea. — In  the  manner  de- 
described  the  sea  has  for  long  periods  been  trimming  the 
edges  of  the  continents  and  islands.  This  may  be  seen  on 
parts  of  Martha's  Vineyard  and  Nantucket  Islands,  along 
the  shores  of  Cape  Ann,  and  everywhere  on  the  sea  border 
of  Maine.  The  coasts  of  Great  Britain  afford  famous  illus- 
trations. The  south  shore  of  Mull  displays  magnificent 
sea  cliffs  800  feet  high.  These  show  large  trespass  of  the 
sea  upon  the  ancient  area  of  the  land.  Staffa  belongs  to  a 
groiTp  of  islets  which  are  remnants  of  an  extended  forma- 
tion, and  now  stand  as  weird  sentinels  amid  the  waters. 


121  GEOLOGY 

The  shores  of  Scotland  abound  in  such  displays  of  the  onset 
of  the  sea ;  such  are  the  spectral  islands  on  the  north, 
where  the  cliffs  of  Hoy  rise  1,200  feet  above  the  waters. 
The  east  of  England  has  lost  extensively  during  the  pres- 
ent century.  Here  the  waste  is  rapid,  owing  to  the  destruc- 
tible character  of  the  rocks,  which  commonly  are  uncon- 
solidated  sands  and  clays.  The  landward  march  of  the  sea 
is  sometimes  several  yards  per  year,  and  not  only  farms  but 
a  number  of  villages  have  been  destroyed.  As  we  have 
seen,  this  loss  is  in  a  general  way  compensated  by  the  re- 
clamation of  the  fen  lands.  On  the  north  and  west  of 
England  and  along  the  shores  of  Scandinavia  the  loss  is  less 
rapid,  because  the  rocks  are  strong. 

99.  Limitations  of  marine  erosion. — If  by  any  means  cliffs 
have  been  formed  which  descend  directly  into  water  of 
some  depth,  the  waves  have  little  power.  Fragments  of 
rock  sink  at  once  to  the  bottom,  below  the  level  of  effective 
wave  action,  and  therefore  can  not  be  used  as  weapons  of 
destruction.  If  the  land  keeps  a  fixed  position  relative  to 
sea  level,  encroachment  of  the  sea  upon  land  must  be  lim- 
ited to  a  few  miles.  Erosion  can  only  be  carried  to  slight 
depths,  and  the  waters  of  the  new  marginal  belt  must  be 
shallow.  The  waves  of  the  open  sea  can  not  be  powerfully 

transmitted  through 
such  waters  to  long 
distances. 

100.  Beaches. — 
The  term  beach  is 
applied  to  the  nar- 
row belt  of  rock  or 
of  f  ragmental  mate- 

Fio.  61. -Action  of  the  sea  on  a  steep  shore  line.         •    i  i  •    i      v         •. 

Cliff  with  wave-cut  and  wave-built  platform.-      mls    wh.lch    ll6S   be~ 
After  GILBERT.  tween  high  and  low 

tide.  It  is  relative- 
ly wide  or  narrow,  with  a  slight  or  considerable  slope,  ac- 
cording to  the  special  conditions  under  which  it  is  formed. 


THE  OCEAN  125 

It  may  have  a  rocky  floor  if  the  force  and  direction  of  the 
waves  and  shore  currents  are  such  as  to  sweep  off  the  mate- 
rials of  erosion.  Commonly  it  consists  of  sand,  gravel,  and 
bowlders.  These  fragments  will  be  angular  if  they  have 
been  recently  broken  from  the  parent  mass,  but  well  beaten 
and  rounded  if  they  have  traveled  far  or  have  long  been 
ground  in  the  mill  of  the  surf. 

Thus  beach  deposits  are  usually  coarse  because  only  such 
materials  can  stay  in  a  zone  of  vigorous  water  action.  The 
fine  sands  are  all  carried  out  and  deposited  at  the  bottom 
of  deeper  and  more  quiet  waters.  The  rounding  of  beach 
pebbles  is  due  to  their  incessant  rubbing  upon  each  other 
and  the  blows  which  they  receive  in  violent  wave  action. 
In  the  reflux  of  a  wave  from  the  shore,  the  hoarse  grinding 
of  stones  upon  each  other  may  be  heard.  They  are  carried 
in  and  out  by  long,  sharp  zigzags  with  each  onset  and  re- 
tirement of  the  waves.  If  there  be  also  a  prevailing  shore 
current  this  tendency  will  be  added,  and  as  a  result  the 
fragments  may  travel  alongshore  for  many  miles.  Hounded 
fragments  of  brick  have  been  found  nine  miles  from  the 
yard  where  they  were  made  on  the  shore  of  Martha's  Vine- 
yard. But  the  total  transport  of  the  fragments  may  have 
amounted  to  some  scores  of  miles.  Seaweeds  often  attach 
themselves  to  fragments  of  rocks  and  to  shells.  The  sea- 
weeds are  forcibly  attacked  by  waves,  and  Professor  Shaler 
has  pointed  out  that  large  quantities  of  rock  are  thus 
dragged  ashore  during  storms. 

101.  Pocket  beaches. — Sand  beaches  often  lie  between 
stretches  of  rocky  shore  line.  The  rocks  are  broken  in 
pieces  by  wave  action,  migrate  alongshore,  and  are  gradu- 
ally ground  into  fine  material,  which  is  deposited  in  bays 
or  inlets.  Such  sandy  shores  are  often  concave  in  outline 
toward  the  sea,  and  have  been  called  pocket  beaches.  If 
the  supply  of  waste  is  abundant,  the  curve  approximates  a 
straight  line;  if  meager,  it  may  recede  deeply  landward. 
If  the  waste  is  migrating  alongshore  in  a  fixed  direction, 


126  GEOLOGY 

it  may  consist  of  coarse  gravel  and  bowlders  at  one  end 
and  of  sand  only  at  the  other  end  of  the  beach.  The  gen- 
eral effect  of  the  ocean  is  to  cut  away  headlands  and  fill 


FIG.  62.— Shore  cliff  and  pocket  beach  of  coarse  gravel, 
Long  Bench,  Marblehead  Neck,  Mass. 

up  depressions,  thus  straightening  the  shore  line,  or  form- 
ing a  uniform  and  broad  curve. 

102.  Offshore  shoals  and  islands. — If  the  sea  bottom  de- 
scends gently,  the  water  will  be  shallow  for  some  distance. 
One  consequence  is  that  strong  waves  will  break  offshore. 
When  the  wave  breaks  and  subsides,  it  deposits  its  load, 
and  thus  a  shoal  forms.  This  may  soon  become  an  island 
whose  sands  are  raised  into  hillocks  by  both  wind  and 
wave.  Between  the  island,  which  is  often  elongated  and 
parallel  with  the  shore,  is  a  belt  of  quiet  water,  or  lagoon. 
It  is  possible  for  small  boats  to  navigate  such  protected 
waters  along  much  of  the  New  Jersey  coast  and  other 


THE  OCEAN 


127 


large  portions  of  our  Atlantic  shore  line.  These  lagoons 
tend  to  fill  up  with  vegetable  remains  and  the  wash 
brought  in  by  streams,  and  thus  in  time  the  islands  be- 
come a  part  of  the  continental  surface.  On  a  sinking 


FIG.  63. — A  straightened  shore  line,  south  shore  of  Martha's  Vineyard. 
Contour  interval,  20  feet. 


shore  line  this  result  is  ever  delayed.  On  a  rising  shore 
line  it  is  hastened.  Many  tidal  marshes  are  made  in  this 
manner,  and  form  a  stage  in  such  growth  of  the  land.  As 
we  have  already  seen,  the  material  deposited  at  the  mouth 
of  rivers  may  either  form  a  typical  delta,  or  a  bar  opposite 
the  mouth  of  the  stream,  or  may  be  widely  distributed, 
depending  upon  the 
strength  of  the  tides 
and  the  direction  and 
power  of  the  shore 
currents.  Materials 
brought  from  remote 

parts  of  the  shore  by  such  currents  may  be  carried  more  or 
less  completely  across  the  mouth  of  bays  and  inlets,  form- 
ing bars  or  hooks.  Currents  may  also  so  meet  as  to  form 
sharp  projecting  beaches  or  cusps. 

103.  Ice  and  the  sea. — In  arctic  latitudes  frozen  water 
assumes  importance  as  a  marine  agent.  Along  the  shore  a 
mass  of  ice  of  considerable  width  and  with  a  thickness  of 


FIG.  64. — Section  of  barrier  beach  and  lagoon. 
After  GILBERT. 


128  GEOLOGY 

20  feet  or  more  forms,  and  is  called  the  ice  foot.  It  at- 
taches itself  to  the  rocks  and  earth  below,  and  receives 
deposits  from  overhanging  cliffs.  In  the  spring  it  breaks 
up  with  much  rending  and  redistribution  of  rocky  material. 
The  surface  water  of  arctic  bays  freezes  into  massive  sheets 
of  ice,  which  breaks  up  in  summer,  and  forms  floe  ice  or  the 


FIG.  65. — Iceberg  off  the  coast  of  Labrador. 

ice  pack,  entrapping  ships,  piling  up  under  lateral  pressure, 
with  gouging  of  shallow  sea  bottoms,  or  floating  off  for  long 
distances  into  the  open  sea.  Floes  from  Greenland  waters 
may  reach  Labrador  or  Newfoundland.  The  Polaris  party 
of  19  persons  drifted  on  floe  ice  for  2,000  miles,  between 
October  15,  1872,  and  April  29th  of  the  following  spring. 
Glaciers  entering  the  sea,  as  in  Greenland,  Spitzbergen, 
and  Alaska,  and  the  antarctic  region,  give  off  masses  of 


THE  OCEAN  129 

great  thickness  and  bulk,  which  traverse  the  seas  as  ice- 
bergs. These  are  a  geological  agent  of  considerable  im- 
portance and  interest.  The  smaller  bergs  may  be  formed 
by  masses  cracking  off  from  the  glacier  front  and  falling 
into  the  water.  The  larger  bergs,  however,  are  buoyed  up 
as  a  thick  glacier  enters  deep  water,  and  thus  they  sail 
away.  They  may  rise  500  feet  or  more  out  of  the  water, 
but  as  ice  is  nearly  as  heavy  as  water,  only  a  fraction  of  the 


FIG.  66. — An  iceberg  which  has  shifted  its  plane  of  equilibrium. 

bulk  lies  above  the  surface.  The  relative  height  above  and 
below  vary  much  with  the  shape,  ranging  from  ^  to  ^ 
above  water.  Some  icebergs  are  1  or  2  miles  in  length. 
Fleets  of  them  sometimes  invade  the  temperate  latitudes 
of  the  Atlantic,  chilling  the  waters  and  endangering  navi- 
gation. By  progressive  melting  their  forms  often  become 
rugged  and  mountain-like.  Many  icebergs  derived  from 
the  antarctic  ice  sheet  are  said  to  be  tabular  in  form. 

Icebergs  moving  in  shallow  waters  may  grind  and  erode 
the  sea  bottoms,  but  not  to  an  important  degree.  Their 
chief  geological  efficiency  is  by  way  of  transportation.  On 


130  GEOLOGY 

their  surface,  or  more  commonly  frozen  within  and  at  the 
base,  are  bowlders  and  masses  of  earth,  which  may  be  car- 
ried across  many  degrees  of  latitude  or  longitude  and 
dropped  in  remote  seas.  Thus  the  North  Atlantic  receives 
every  summer  a  contribution  from  the  arctic  lands.  If  a 
glaciated  pebble  should  be  found  in  rocks  formed  in  a  deep 
sea,  it  would  not  therefore  prove  that  there  had  been  a 
glacier  at  that  point ;  it  would  only  show  that  somewhere 
a  glacier  had  invaded  the  sea  border. 

THE  DEEP  SEAS 

104.  We  have  thus  far  referred  briefly  to  the  great  law 
that  coarser  land  waste  finds  lodgment  near  the  shore  in 
comparatively  shallow  waters,  while  the  fine  muds  come  to 
rest  at  greater  depths.     Organic  remains  also  are  deposited 
everywhere  on  the  floor  of  the  ocean.     We  shall  now  give 
a  brief  account  of  the  materials  of  the  sea  bottom.     The 
conformation  of  ocean  basins  will  be  more  appropriately 
described  in  Chapter  XIV. 

The  investigation  of  deep-sea  deposits  is  modern.  Dur- 
ing the  past  fifty  years  expeditions  have  gone  forth  with 
special  equipment  for  observing  the  phenomena  and  gath- 
ering the  sediments  of  the  abyssal  seas.  Among  the  most 
important  of  such  researches  are  those  made  by  the  United 
States  Coast  Survey,  particularly  upon  the  Gulf  Stream. 
The  depth  and  extent  of  the  oceans  render  it  difficult 
to  gain  adequate  knowledge  of  them.  It  has  been  well 
observed  that  if  the  continents  were  deeply  covered 
with  waters,  such  conspicuous  features  as  Mount  Etna 
or  the  Grand  Cafion  of  the  Colorado  might  long  escape 
observation. 

105.  Means  of  research. — One  of  the  most  important  of 
these  is  the  sounding  line,  which  consists  of  a  steel  wire 
with  a  self-detaching  weight.     The   dredge    is    a    scoop 
which  brings   up  a  quantity  of   mud   from   the   bottom, 


THE  OCEAN  131 

with  the  organic  remains  and  living  forms  which  it  may 
hold.  Attached  to  the  dredge  is  a  fringe  which  as  a  net 
serves  to  entangle  certain  branching  forms  and  tow  them 
to  the  surface.  Similarly  a  drag  is  arranged  which  moves 
along  the  surface  of  the  sea  and  gathers  the  creatures  who 
live  at  that  horizon.  By  means  of  self-registering  ther- 
mometers the  temperature  of  the  sea  at  all  depths  is  ascer- 
tained, and  means  are  also  found  for  collecting  samples  of 
the  lower  waters  for  chemical  examination. 

The  various  groups  of  marine  animals  found  are  pre- 
served and  submitted  to  specialists  for  deliberate  study, 
and  thus  in  time  a  minute  knowledge  of  the  sea  realm  in 
all  its  parts  will  be  gathered. 

106.  Sea-bottom  deposits. — The  bulk  of  the  land  waste 
is  deposited  on  the  landward  side  of  the  100-fathom  line. 
Below  and  beyond  this  line  is  a  transitional  zone  in  which 
the  deposits  are  of  fine  mud,  but  terrigenous — that  is,  con- 
taining particles  which  can  be  recognized  as  of  land  origin. 
These  muds  consist  of  blue,  green,  and  red  marls.  They 
may  be  found  at  great  depths  if  the  region  be  not  too  far 
from  land.  The  color  and  composition  of  such  deposits 
vary  with  the  character  of  the  neighboring  lands. 

At  still  greater  but  not  the  greatest  depths  are  fine 
muds,  to  which  the  general  name  ooze  is  given.  These  con- 
sist largely  of  the  entire  or  dissolved  shells  of  minute  organ- 
isms, and  receive  special  names  from  the  kind  of  organism 
which  predominates.  They  have  been  studied  in  an  inter- 
esting manner  by  Murray  and  others.  Portions  of  the  mud 
were  mixed  with  a  glue  and  dried.  Small  chips  of  this 
rocklike  mass  were  then  ground  so  thin  as  to  be  trans- 
parent, after  the  usual  manner  of  making  rock  sections. 
Much  of  this  mud  is  called  Oldbig&rina  Ooze,  from  the 
great  number  of  shells  of  this  sort.  The  Globigerina  is  a 
minute  jellylike  creature  which  secretes  shells  of  carbonate 
of  lime.  It  lives  in  the  upper  waters  of  the  sea,  and  its 
shells  in  infinite  numbers  rain  down  upon  the  sea  bottoms. 


132 


QEOLOGY 


Other  deep-sea  muds  consist  largely  of  Eadiolarians,  which 
secrete  shells  of  marvelously  graceful  and  elaborate  forms 
out  of  silica.  Such  muds  are  called  Radiolarian  Ooze. 
The  siliceous  cases  of  the  microscopic  plants  called  Dia- 
toms may  also  give  character  and  name  to  these  deposits. 


FIG.  67.-C 


seen  in  thin  i 
After  AGASSIZ. 


At  still  greater  depths,  upon  the  abyssal  sea  bottoms,  lie 
sheets  of  Red  Clay.  These  extend  over  very  great  areas, 
and  are  variously  composed.  They  are  believed  to  result 
largely  from  the  decomposition  of  volcanic  dust  and  pum- 
ice, which  falls  or  is  floated  extensively  over  the  seas,  and 
sinks  at  length  to  the  bottom.  Other  particles  are  found 
which  are  believed  to  be  fragments  of  meteorites.  Concre- 
tions of  manganese  occur,  also  shells,  shark  teeth,  and  other 
organic  remains.  These  muds  accumulate  very  slowly,  and 
are  unlike  any  rocks  now  forming  a  part  of  the  lands. 
This  goes  far  to  prove  the  permanence  of  oceans  and  con- 
tinents, for  the  extensive  marine  rocks  of  the  continents 
must  have  been  formed  in  waters  of  moderate  depth. 

107.  Life  of  the  deep  seas.— Here  we  have  a  great  body 
of  facts  into  which  it  is  not  now  possible  to  enter.  It  must 


THE  OCEAN  133 

suffice  to  observe  that  each  chief  group  or  type  of  marine 
animals  has  some  representatives  at  profound  depths.  The 
abysses  of  the  sea  are  not  therefore  barren,  although  they 
are  less  populous  than  the  upper  waters.  It  is  not  easy  to 
picture  the  conditions  of  life  which  there  reign.  Thus 
almost  complete  darkness  prevails ;  hence  some  deep-sea 
fishes  have  large  eyes,  others  have  none.  The  supply  of 
air  is  small,  hence  vitality  must  be  low.  There  is  no  day, 
no  night,  and  no  change  of  seasons.  All  creatures  there 
exist  under  vast  pressure,  and  it  is  the  one  environment 
which  may  remain  without  essential  change  throughout 
geological  ages.  Hence  we  are  apt  to  find  there  living 
forms  which  are  nearly  identical  in  structure  with  those 
found  fossil  in  the  ancient  rocks. 


10 


CHAPTEE  VIII 

VOLCANOES 

108.  WE  may  well  begin  our  study  with  an  account  of  a 
small  but  familiar  example,  Vesuvius.  Its  cone  rises  but 
about  4,000  feet  above  the  sea,  and  is  small,  both  in  height 
and  bulk,  as  compared  with  Etna,  or  many  volcanoes  of 
South  America,  Mexico,  or  the  Pacific  Ocean.  But  since 
classic  times  its  history  is  better  known  than  that  of  any 
other  volcano,  and  it  has  therefore  a  peculiar  interest.  It 
rises  from  the  Bay  of  Naples,  and  is  ever  within  sight  of 
the  famous  city,  to  whose  scholars  we  are  indebted  for 
much  knowledge  of  its  operations.  For  many  years  an 
observatory  has  been  maintained  on  the  mountain  and 
the  university  has  gathered  an  extensive  library  relating 
to  volcanic  and  earthquake  phenomena.  Pompeii  lies  at 
its  base  on  the  south  and  Herculaneum  on  the  west,  while 
across  the  bay  are  Baiae,  Avernus,  Misenum,  and  other 
localities  known  in  classic  literature. 

The  first  eruption  of  Vesuvius  in  historic  times  was  that 
of  the  year  79  A.  D.,  by  which  Pompeii  and  Herculaneum 
were  overwhelmed.  Eruptions  had  taken  place  on  the  is- 
land of  Ischia  in  times  of  the  Greek  settlements,  and  these 
may  have  served  by  way  of  a  safety  valve  for  Vesuvius.  In 
the  year  63  B.  c.  Spartacus  and  his  band  had  taken  refuge 
in  its  crater.  In  63  A.  D.,  and  following  years,  earthquakes 
had  occurred  which  injured  buildings  in  Pompeii.  In  the 
year  79  came  the  great  catastrophe,  a  series  of  explosions 
by  which  vast  quantities  of  volcanic  ashes  (so  called)  were 
134 


FIG.  68.— Eruption  of  Vesuvius,  1878. 


^36  GEOLOGY 

ejected.  It  is  important  to  remember  that  this  eruption 
was  without  flows  of  lava.  Pliny,  the  Elder,  the  famous 
Koman  naturalist,  was  then  commander  of  a  fleet  stationed 
across  the  bay,  at  Misenum.  He  went  over  the  waters  to 
see  the  eruption  more  favorably,  and  in  part,  as  it  is  said, 
to  rescue  a  friend.  There  he  was  suffocated  by  the  fumes 
and  ash  of  the  volcano.  This  event  is  described  with  great 
vividness  by  his  nephew  in  letters  written  to  Tacitus  the 
historian,  and  thus  we  have  good  information  concerning 
the  eruption. 

The  characteristic  phenomena  were  the  vast  clouds  of 
vapor  pouring  forth,  often  taking  the  form  of  a  pine  tree, 
with  brilliant  flashes  of  light  alternating  with  midnight 
darkness,  the  falling  of  a  great  sheet  of  fragmental  mate- 
rial upon  the  ground  and  upon  the  waters  of  the  harbor, 
with  rockings  of  the  earth  and  recession  of  the  sea.  Pom- 
peii was  buried  in  the  ash,  and  was  only  rediscovered  in 
1748.  Herculaneum  also  was  covered  by  ash  and  volcanic 
mud. 

After  various  minor  outbreaks,  another  great  eruption 
occurred  in  1036,  and  this  affords  the  first  sure  record  of 
lava  flows  from  Vesuvius  in  historic  times.  Other  erup- 
tions occurred  in  1138,  1306,  and  1631.  In  1538,  how- 
ever, on  the  site  of  the  Lucrine  Lake,  across  the  bay  of 
Naples,  a  hill  now  called  Monte  Xuovo  was  formed  by  vol- 
canic agency,  being  reared  to  a  height  of  440  feet  within  a 
week.  In  the  terrible  outburst  of  1631,  18,000  persons  per- 
ished and  the  skies  were  darkened  as  far  as  Constantinople. 
Eruptions  have  been  frequent  since  that  time.  In  1822 
the  cone  had  attained  a  height  of  4,200  feet,  and  the  crater 
was  nearly  full.  Its  contents  were  blown  out,  leaving  it 
1,000  feet  deep,  while  the  height  of  the  cone  was  brought 
down  to  3,400  feet.  A  segment  of  the  ancient  rim,  which 
was  broken  by  the  explosions  of  79,  still  remains  on  the 
southeast,  and  is  called  Monte  Somma. 

Thus  the  important  facts  about  Vesuvius  are  its  cone 


VOLCANOES  13Y 

of  ejected  matter  with  its  many  changes  of  height  and 
form,  its  explosive  and  more  quiet  eruptions  of  water, 
vapor,  ash,  and  molten  material,  and  the  irregular  and 
intermittent  character  of  its  activity.  We  note  also  the 
presence  of  earthqake  shocks  and  of  volcanic  phenomena 
in  the  vicinity.  We  are  now  better  prepared  for  a  short, 
orderly  statement  of  the  principles  of  vulcanism,  as  this 
department  of  geology  is  sometimes  called.  We  shall  then 
follow  with  an  account  of  some  other  volcanoes  whose 
points  of  likeness  and  difference  are  instructive. 

PKINCIPLES  OF  VOLCANIC  ACTION 

109.  Essential  features. — These  are  an  opening  in  the 
earth's  crust  and  a  discharge  of  heated  materials.     Other- 
wise the  greatest  diversity  prevails.     Thus  a  cone  or  moun- 
tain mass  is  no  necessary  part  of  a  volcano,  though  numer- 
ous volcanoes  do  form  such  a  structure.     Many  facts  go  to 
prove  that  the  most  important  eruptions  in  the  history  of 
the  globe  have  taken  place  through  fissures,  whence  liquid 
rocks  have   spread   widely  without  forming  mountainous 
masses. 

110.  Active,   dormant,   and  extinct  volcanoes. — Here  110 
sharp  division  is  possible.     Who  shall  say  whether  the  ab- 
sence of  eruption  for  a  hundred  or  a  thousand  years  is 
enough  to  classify  a  volcano  as  dormant  ?    Before  79  A.  D. 
the  surrounding  people  might  naturally  have  considered 
Vesuvius  extinct.     Still,  the  distinction  is  a  convenient  one 
for  description.     Some,  like  Stromboli,  are  clearly  active, 
while  many  others,  as  we  shall  see  in  later  pages,  are  as 
surely  extinct. 

111.  Ejected  materials. — These  are  of  four  classes :  (1) 
Gases.     (2)  Water.     (3)   Lavas.      (4)  Fragmental  matter. 
We  take  these  in  their  order. 

112.  Gases  issuing  from  volcanoes. — Chief  among  these  is 
the  vapor  of  water.     According  to  an  estimate  quoted  by 
Geikie,    ^  of  the  cloud  which  rises  over  a  volcano  consist 


13g  GEOLOGY 

of  steam.  But  many  other  gases  are  disengaged  in  the 
subterranean  region  by  the  great  heat  applied  to  the  rocks, 
and  the  variety  of  combinations  and  chemical  reactions 
which  result.  Among  these  are  hydrochloric  acid,  hydro- 
gen and  various  of  its  compounds,  and  carbon  dioxide. 
Exhalations  of  the  last  in  volcanic  regions  sometimes  cause 
the  suffocation  of  animals,  while  the  presence  of  hydrogen 
may  give  origin  to  true  flames.  It  is  to  be  observed,  how- 
ever, that  the  appearance  of  flame  in  eruptions  is  usually 
caused  by  light  from  molten  lavas  illuminating  the  rising 
clouds  of  vapor.  From  the  erupted  gases  various  sub- 
stances accumulate.  Such  are  sulphur,  alum,  common  salt, 
and  others,  which  may  gather  in  sufficient  amounts  to  have 
economic  value. 

113.  Water  ejected  from  volcanoes. — We  have    already 
seen  that  much  water  is  produced  by  the  condensation  of 
heated  vapors.     It  may  also  be  formed  directly  by  the  melt- 
ing of  snows  lying  on  the  mountain  previous  to  an  erup- 
tion, or  by  the  melting  of  ice,  as  of  glaciers,  or  masses  of 
ice  long  buried  under  slopes  of  rock  waste.     The  craters  of 
dormant  vents  may  become  lake  basins,  whose  waters  are 
invaded  when  volcanic  activity  is  resumed.     We  must  in 
addition  take  account  of  the  water  long  buried  and  incor- 
porated with  the  rocks  of  the  crust,  which  vaporizes  with 
the  melting  of  the  inclosing  mass.     The  water  of  eruptions 
becomes  geologically  significant  when  it  mixes  with  the 
friable  ash  of  explosive  eruptions  and  produces  torrents 
of  mud. 

114.  Lavas.— Lava  streams  are  commonly  thought  to  be 
the  characteristic  product  of  volcanoes,  but  it  is  doubtful 
whether  they  are  as  important  as  the  materials  described 
under  the  next  head.     (See  section  118.)     It  is  convenient 
to  study  lavas  under  several  subdivisions. 

115.  Composition  and  appearance  of  lavas.— If  lavas  con- 
tain a  large  percentage  of  iron  or  other  metallic  bases,  they 
are  said  to  be  basic.     If,  however,  they  consist  largely  (60 


VOLCANOES  139 

to  80  per  cent)  of  silica  or  quartz,  they  are  called  acidic. 
Basalts  are  basic,  while  obsidian,  trachyte,  and  rhyolite  are 
acidic.  Obsidian  is  a  volcanic  glass.  Between  the  two 
typical  extremes  all  gradations  are  found.  If  lavas  cool 
slowly,  they  may  have  a  highly  crystalline  structure.  If 
they  cool  rapidly,  they  will  be  structureless  and  glassy. 
They  are  porous,  or  massive  and  compact,  according  to  the 
amount  of  steam  or  other  vapor  contained  in  them  while 
they  are  cooling.  If  the  amount  is  large,  the  lava  may  con- 
tain many  spherical  or  elongated  air  pockets,  and  may  be 
so  full  of  small  cavities  of  this  nature  as  to  form  a  pumice 
stone,  which  is  used  for  polishing.  It  is  often  so  light  as 
to  float  in  great  quantities  on  the  surface  of  the  sea.  The 
color  of  lavas  is  varied.  Basic  lavas  are  commonly  dark, 
while  the  acidic  are  often  of  light  colors.  Obsidian  may  be 
of  a  transparent  blue,  or  may  be  even  black.  In  the  Yel- 
lowstone Park  the  lavas  of  ancient  eruptions  have  many 
hues,  in  brilliant  variety. 

116.  Flow  of  lavas. — The  appearance  of  a  stream  of  lava 
varies  with  the  character  of  the  material  and  the  form  of 
the  ground.     Eapids  and  cataracts  may  occur,  and  lakes 
are  formed  behind  barriers.     The  surface  often  has  a  ropy 
appearance.     The  cooling  upper  lava  is  crumpled  as  the 
still  liquid  lower  portions  push  on.     Often,  in  a  similar 
manner,  the  surface  parts  are  completely  broken  up,  and 
the  stream  looks  like  a  creeping  mass  of  jagged  and  black- 
ened bowlders.     The  slag  from  a  blast  furnace  illustrates 
many  of  the  features  of  lava  streams.     Some  lavas  are 
much  more  liquid  than  others,  and  this,  with  different  in- 
clinations of  slopes,  causes  great  differences  in  the  rate  of 
flow.     The  lava  of  Vesuvius,  in  an  eruption  of  1805,  flowed 
3f  miles  in  four  minutes,  then  much  more  slowly.     A  lava 
stream  may  creep  for  months. 

117.  Cooling  of  lavas. — The  surface  cools  rapidly,  form- 
ing a  crust  over  which  one  may  walk,  even  when  the  under 
portions  are  intensely  hot.     Lava  is  a  poor  conductor,  and 


VOLCANOES  141 

the  rate  of  cooling  of  large  masses  is  slow.  The  lava  issu- 
ing from  Etna  in  1787  was  still  steaming  in  1830. 

118.  Fragments!  materials. — Volcanoes  make  an  immense 
contribution  to  the  surface  of  the  lands,  and  add  largely  to 
the  formation  of  the  sea  bottom  by  ejection  of  fragmental 
matter.  Indeed,  we  should  not  think  of  volcanoes  (and 
the  same  might  be  said  of  glaciers)  as  abnormal  or  excep- 
tional parts  of  the  economy  of  the  earth.  Such  material 
is  largely  at  first  in  a  molten  state,  blown  into  dust  and 
bits  of  larger  size  by  stupendous  explosions.  This  is  the 
origin  of  the  so-called  volcanic  ashes,  which  are  in  no  sense 
products  of  combustion,  but  rather  of  intense,  mechan- 
ically applied  energy.  So  great  is  the  quantity  of  such 
matter  that  the  air  may  be  darkened  for  hundreds  of  miles, 
and  the  surface  of  the  sea  covered  for  long  distances  from 
the  seat  of  the  eruption.  Darkness  prevailed  to  a  distance 
of  35  miles  from  one  of  the  Nicaraguan  volcanoes,  and  24 
miles  away  the  ash  fell  to  a  depth  of  10  feet.  Dust  from 
an  Iceland  volcano  has  fallen  in  such  quantities  as  to  be 
shoveled  from  the  decks  of  ships  near  the  Orkney  and 
Shetland  Islands,  while  in  1783  ash  from  the  same  source 
fell  on  Caithness,  the  northeast  county  of  Scotland,  so 
largely  as  to  destroy  crops.  The  season  was  remembered 
as  the  year  of  "  The  Ashie." 

If  the  fragments  are  of  the  size  of  a  pea  and  ranging  to 
that  of  a  walnut,  they  are  called  lapilli.  Sometimes  huge 
blocks,  either  of  the  older  and  hardened  lava,  or  of  the 
country  rock  through  which  the  vent  is  formed,  are  hurled 
to  some  height  and  distance.  Lumps  of  molten  lava  are 
also  cast  into  the  air  and  cool  while  under  the  influence  of 
rotary  motion,  assuming  a  variety  of  pear-shaped  and  disk- 
like  forms.  These  are  known  as  volcanic  bombs  and  vary 
in  diameters  from  an  inch  or  two  to  one  or  more  feet. 

The  ejected  vapor  gives  origin  to  powerful  rains,  and 
the  water  mixes  with  the  volcanic  ash,  often  in  mud  flows 
Q£  considerable  extent.  Such  muds,  or  the  ash  consolidat- 


142  GEOLOGY 

ing  in  the  place  where  it  has  fallen,  makes  the  volcanic  rock 
known  as  tufa,  of  which  there  are  extensive  formations  in 
some  volcanic  regions.  It  is  porous,  but  may  be  quite  firm 
and  be  used  as  a  building  stone.  Subterranean  dwellings, 
as  in  Naples,  are  sometimes  excavated  in  it. 

119.  Volcanic  cones. — Some  volcanic  eruptions  are  at- 
tended by  powerful  explosions,  with  little  flow  of  lavas.  A 
rim  is,  however,  formed  of  dislodged  fragments  of  the 
country  rock.  Such  cones  are  low  and  broad,  and  are 
illustrated  in  some  extinct  volcanoes  of  Italy  and  the 
Khine.  A  typical  cone  has  a  strong  slope  and  a  consider- 
able height  in  relation  to  the  area  covered  by  its  base.  It 


FIG.  70. — Volcanic  cones  composed  of  fragmental  material,  and  breached  on  one 
side  by  an  outflow  of  lava. 

is  not,  as  was  once  thought,  due  to  the  bulging  up  of  the 
torn  edges  of  the  country  rock,  but  is  simply  a  pile  of  lava 
and  ash  gathered  around  the  opening.  Some  cones  are 
built  entirely  of  tufa,  or  consolidated  ash  ;  others,  though 
few  in  number,  of  lava ;  while  many,  like  Vesuvius,  show 
beds  of  tufa  alternating  with  streams  of  lava.  The  mate- 
rials of  one  eruption  are  gradually  buried  by  those  of  later 
outbreaks,  and  meanwhile  surface  streams  spread  the  waste 
more  widely  at  the  base.  Gorges  cut  by  streams  of  water 
may  later  be  filled  with  streams  of  lava,  and  the  shocks  of 
explosions  shiver  the  cone  and  cause  rents  or  fissures  into 
which  the  lavas  flow.  These  dikes  or  walls  of  cooled  lava 
may  intersect  the  mass  in  great  numbers,  serving  as  girders. 
If  a  cone  be  built  wholly  of  ash  it  will  be  fragile,  and  a 
later  flow  of  lava  may  tear  away  one  side  of  it.  Such 


VOLCANOES  143 

breached  cones  are  found  among  the  extinct  volcanoes  of 
central  France  (see  Fig.  70).  Cones  rise  not  only  from  the 
land,  but  the  sea  bottom,  like  those  of  Hawaii  and  many 
other  volcanoes  of  the  oceanic  islands.  Some  such  cones 
fail  to  attain  the  surface,  and  are  therefore  wholly  due  to 
submarine  eruptions.  Others,  as  Vesuvius  and  Etna,  are 
believed  to  have  begun  as  submarine  vents  opening  on  a 
sea  floor  which  has  since  been  raised  above  the  surface  of 
the  waters. 

DECADENT  VULGARISM 

120.  After  powerful    eruptions   cease,  minor  volcanic 
activities  remain,  to  which  this  general  name  may  be  given. 
We  refer  to  the  issues  of  gas,  mud,  and  heated  waters, 
which  may  continue  even  when  the  volcanic  energies  of  a 
region  have  become  nearly  extinct.     An  opening  through 
which  gases  arise  is  called  afumarole  (Latin  fumus,  smoke). 
Hot  springs  are  often  formed  when  subterranean  waters 
come  in  contact  with  heated  rocks.     Deposits   of  rocky 
matter  from  such  springs  are  not  extensive,  but  may  be 
interesting,  owing  to  their  characters  and  form. 

121.  Geysers. — These  are  a  most  striking   product   of 
volcanic  forces.     Le  Conte  has  well  denned  them  as  period- 
ically eruptive  springs.     They  issue  from  well-like  pools  of 
great  depth,  and  at  more  or  less  regular  intervals  send  up 
powerful  jets  into  the  air.     These  play  usually  for  a  few 
moments  only,  forming  magnificent  fountains  and  sending 
off  clouds  of  steam.     The  waters  then  subside  and  remain 
quiet  during  periods  varying  from  a  few  minutes  to  several 
hours. 

Three  geyser  regions  are  known — Iceland,  New  Zealand, 
and  the  Yellowstone.  The  diameter  of  the  jets  in  the  last 
region  varies  from  2  to  20  feet  and  their  heights  range  to 
200  feet.  The  Grand  Geyser  plays  twenty  minutes,  the 
Giant  three  hours.  The  Grand  Geyser  erupts  but  once  in 
thirty-two  hours,  and  Old  Faithful  at  brief  intervals.  The 


g 


PIG.  71  .-Old  Faithful  Geyser,  Yellowstone  National  Park,  Wyoming. 


VOLCANOES  145 

heated  waters  may  take  silica  into  solution,  which,  owing 
to  cooling  and  relief  from  pressure,  is  deposited  about  the 
geyser  pool. 

The  heat  of  the  water  is  derived  from  subterranean 
lavas  of  late  geological  date,  which  retain  much  heat. 
Thus  the  Yellowstone  Park  was  the  scene  of  powerful 
eruptions,  as  shown  by  its  extensive  formations  of  lava  and 
its  tall  volcanic  mountains.  The  periodical  flow  is  ex- 
plained as  follows :  The  water  rises  through  a  narrow  pas- 
sage of  great  depth.  The  waters  far  below  become  very 
hot,  while  those  near  the  surface  remain  comparatively 
cool,  since  there  is  not  room  for  free  circulation,  such  as 
takes  place  when  water  is  heated  in  an  ordinary  vessel. 
The  lower  waters  reach  the  boiling  point,  which  indeed  is 
raised  by  the  great  pressure ;  but  at  last  they  flash  into 
steam,  producing  explosions  by  which  the  overlying  col- 
umn of  water  is  forced  out,  as  already  described.  At 
length  the  steam  is  exhausted,  the  waters  subside,  and  the 
quiet  heating  goes  on  again,  making  ready  for  another 
explosion.  Similar  results  have  been  reached  by  experi- 
ments in  the  laboratory. 

DISTRIBUTION  OF  EXISTING  VOLCANOES 

It  must  be  remembered  that  we  here  omit  reference  to 
many  regions  in  which  volcanoes  have  been  active  even  late 
in  the  history  of  the  earth,  and  confine  ourselves  to  regions 
where  such  phenomena  have  taken  place  within  the  mem- 
ory of  men. 

122.  The  Mediterranean  region. — Here  we  have  Vesuvius 
and  the  Lipari  group,  to  which  the  ever-active  Stromboli 
belongs.  These  islands  contain  but  11  square  miles,  but  are 
inhabited  by  12,000  people.  Water  is  scarce,  owing  to  the 
porosity  of  the  soil.  Pumice  stone  is  an  export,  and  the 
crater  of  Vulcano  was  bought  by  a  Scotch  firm  for  its 
product  of  alum,  boracic  acid,  and  sulphur.  Etna  will 
receive  description  by  itself.  Off  the  Sicilian  coast  an 


146  GEOLOGY 

earthquake  was  observed  in  June,  1831.  In  July  a  land 
mass  called  Graham's  Island  emerged  to  the  height  of  200 
feet.  By  the  following  year  the  sea  waves  had  truncated 
the  cone,  forming  a  reef  some  feet  below  the  surface. 


FIG.  72.— Volcanic  district  about  Naples. 

In  the  Grecian  Archipelago  are  the  Santorin  Islands. 
Three  islands  form  a  rude  circuit  about  a  great  caldron 
of  sea  water.  In  the  midst  rises  a  threefold  mountain,  one 
part  of  which  dates  from  186  B.  c.,  with  additions  at  inter- 
vals since  that  time.  The  outer  islands  have  steep  inner 
faces  and  gentle  outward  slopes,  made  up  of  lava  flows.  It 
is  evident  that  these  islands  are  the  fringe  of  a  vast  ancient 
cone,  whose  heart  has  been  blown  out  by  some  prehistoric 
explosive  eruption,  while  activity  has  been  moderately  re- 
sumed during  and  since  classic  times. 

123.  Atlantic  region. — A  chain  of  volcanic  islands  ex- 
tends north  and  south  in  the  Atlantic  Ocean,  though  we 
are  not  to  infer  any  necessary  connection  among  them. 
Thus  we  have  St.  Helena,  Ascension,  Cape  Verd,  the  Ca- 
naries, the  Azores,  and  then,  far  northward,  Iceland  and 
Jan  Mayen.  In  Iceland  we  have  records  for  1,000  years. 


VOLCANOES 


147 


Hecla  is  a  cone  of  moderate  height,  less  than  5,000  feet, 
and  its  earliest  historical  eruption  dates  from  1104  A.  D., 
the  "sand-rain  winter."  In  1783  on  the  same  island  oc- 
curred the  terrific  eruption  of  Scaptar  Jokull.  One  lava 
stream  was  50  miles  long,  12  to  15  miles  wide,  and  100  feet 
deep.  A  river  valley  was  filled  to  a  depth  of  400  to  600 
feet,  the  tributary  valleys  were  flooded  and  villages  destroyed. 


FIG.  73.— A  group  of  lunar  craters. 

The  lava  of  this  outburst  is  estimated  to  have  exceeded  Mont 
Blanc  in  bulk.  A  minor  group  of  volcanoes  belongs  to 
the  West  Indies,  chiefly  east  of  the  Caribbean  Sea. 


148  GEOLOGY 

124.  Circuit  of  the  Pacific. — Here  we  have  the  numerous 
volcanoes   of  the  Andes,  of  Mexico,  and  of  the  western 
United  States,  such  as  Mount  Shasta,  Mount  Kanier,  and 
Mount  Baker,  the  Aleutian  chain,  with  30  to  40  cones, 
Kamtchatka  and  the  Kurile  Islands,  Japan,  the  Philippine 
Islands,  New  Guinea,  Solomon  Islands,  New  Zealand,  and 
the  Balleny  Islands.     Some  volcanoes  also  are  scattered  in 
the  central  Pacific  waters,  such  as  those  of  the  Hawaiian, 
Friendly,  Society,  and  Marquesas  groups.     Volcanoes  also 
occur  on  Madagascar,  Mauritius,  and  at  other  points  in  the 
Indian  Ocean,  while  the  East  Indian  region  is  one  of  the 
most  important  theaters  of  volcanic  energy.     The  Sunda 
Islands  form  an  offshoot  of  the  Pacific  circuit.     Here  the 
single  island  of  Java  has  45  volcanoes,  of  which  28  are 
active. 

More  than  300  active  vents  are  known  at  the  present 
time.  It  can  hardly  have  escaped  the  student's  attention 
that  the  large  majority  of  these  are  upon  islands  or  along 
the  borders  of  continents.  Only  a  few  important  active 
volcanoes,  as  some  in  Mexico,  are  at  any  considerable  dis- 
tance from  the  sea,  and  those  which  fringe  the  continents 
are  commonly  associated  with  great  lines  of  mountainous 
deformation  of  the  earth's  crust. 

SPECIAL  EXAMPLES 

It  will  now  be  usef  ul  to  give  a  short  account  of  some  of 
the  greater  volcanoes.  We  take  for  this  purpose  Etna,  the 
Hawaiian  group,  and  Krakatoa.  These  will  still  further 
illustrate  the  general  principles  of  igneous  action  already 
stated,  and  they  may  also  be  profitably  compared  with  each 
other  and  with  Vesuvius. 

125.  Etna. — Like  Vesuvius,  this  mountain  has  attracted 
the  attention  of  men   since  classical  times.     The  Greek 
philosophers  and  poets  who  dwelt  in  view  of  the  pile  sought 
to  understand  the  mystery  of  its  fires,  and  in  the  present 
century  much  study  and  many  writings  have  been  devoted 


VOLCANOES  149 

to  this  great  sentinel  of  the  Mediterranean.  Etna  is  one  of 
the  most  imposing  of  volcanic  cones,  because  it  rises  from 
the  sea  border,  in  full  relief,  to  a  height  of  about  10,840 
feet.  Nevertheless,  the  diameter  of  its  base,  about  40 
miles,  is  so  great  that  its  average  slopes  are  but  6  to  8  de- 
grees. Etna  began  as  a  submarine  volcano  in  some  pre- 
historic time,  and  since  the  uprising  of  its  sea  floor  the 
major  part  of  the  cone  has  been  built,  forming  a  mass  of  20 
to  30  times  the  bulk  of  Vesuvius.  Eruptions  have  occurred 
throughout  historic  times.  Although  situated  in  a  sub- 
tropical region,  Etna  rises  above  the  forest  zone  to  regions 
of  perpetual  snow.  A  mass  of  ice  on  the  mountain  side 
has  for  many  years  been  covered  with  a  stream  of  lava,  and 
torrents  of  water  that  sometimes  accompany  eruptions  are 
due  to  the  melting  of  the  snows. 

An  important  feature  of  Etna  is  its  minor  cones,  about 
200  in  number.  These  are  formed  on  the  slopes  by  outflows 
through  fissures,  and  some  of  them  attain  a  height  of  700 
feet.  These  subordinate  cones  may  be  either  destroyed  or 
covered  by  later  eruptions.  If  we  could  make  complete 
vertical  sections  of  Etna,  or  could  peel  up  its  successive 
sheets  of  lava  and  ash,  we  might  find  a  great  number  of 
such  cones.  In  1669  six  fissures  appeared  on  parallel  lines, 
one  of  which,  as  quoted  by  Geikie,  extended  for  12  miles 
and  was  2  yards  in  width,  shining  with  its  fiery  lavas.  An- 
other feature  of  Etna  is  the  Val  del  Bove,  a  profound  gorge 
or  basin  east  of  the  present  crater,  believed  to  have  been 
opened  by  a  stupendous  explosive  eruption  and  to  occupy 
the  place  of  a  former  vent. 

126.  The  Hawaiian  volcanoes. — All  the  islands  of  the 
Hawaiian  chain  are  volcanic,  but  Hawaii,  the  largest,  con- 
tains all  the  active  vents,  three  in  number.  Of  these,  Mauna 
Loa,  near  the  center  of  the  island,  is  the  highest,  13,675  feet. 

The  Hawaiian  cones  are  very  flat,  having  slopes  of  1  to 
10  degrees.  In  this  they  contrast  with  such  steep  cones  as 
Mount  Shasta  and  Mount  Ranier  of  the  Pacific  slope.  The 
11 


150 


GEOLOGY 


craters  are  wide,  sunken  fields,  with  steep  walls,  400  to  1,000 
feet  high.  Much  of  the  floor  of  the  craters  is  a  crust  of 
cooling  lava,  with  lakes  of  still  molten  matter,  which  may 
boil  quietly  or  send  up  violent  fountainlike  jets.  From 
time  to  time  masses  of  the  crater  wall  crack  off  and  sink 


=^£  -r 

t^NkC- 

S:  _,  -  ,  --•   ^  •- 


down,  and  thus  have  formed  in  some  places  a  series  of 
gigantic  steps.  These  crater  basins  are  roughly  elliptical 
and  have  diameters  of  2  to  3  miles. 

Previous  to  the  eruptions  of  1832  and  1840,  according 
to  Dana,  the  lava  had  slowly  risen  about  400  feet  in  the 
crater  of  Kilauea.  In  each  case  this  had  occupied  eight  or 
nine  years.  Eruption,  therefore,  must  take  place  not  over 
the  rim  of  the  crater,  but  through  fissures,  and  thus  the 


VOLCANOES  151 

lava  stream  first  appears  at  some  distance  down  the  moun- 
tain side.  The  lavas  are  of  the  more  liquid  sort,  and  often 
flow  for  long  distances.  The  hydrostatic  pressure  of  the 
great  column  of  lava  in  the  conduit  overcomes  the  resist- 
ance of  the  mountain  before  the  lava  can  be  pushed  to  the 
top,  hence  the  fissure  eruptions.  The  lava  stream  from 
Kilauea  in  1840  reached  the  sea,  a  distance  of  30  miles,  and 
formed  a  fall  a  mile  wide  over  the  sea  cliffs.  A  stream 
from  Mauna  Loa  in  1880  threatened  Hilo,  on  the  east  coast, 
33  miles  from  the  crater. 

Hawaiian  volcanoes  furnish  an  instructive  contrast  with 
Vesuvius  in  the  liquidity  of  their  lavas  and  consequent  flat- 
ness of  their  cones,  in  their  wide  caldron  craters  with 
faulted  rims,  and  in  their  more  prolonged  and  quiet  erup- 
tions, predominating  in  molten  rather  than  f  ragmental  prod- 
ucts. Students  who  wish  to  study  this  island  group  more 
fully  should  consult  Dana's  Characteristics  of  Volcanoes  and 
Button's  Report  on  the  Hawaiian  Volcanoes,  the  latter  in 
the  Fourth  Annual  Report  of  the  United  States  Geological 
Survey. 

127.  Krakatoa, — This  volcanic  island  lies  in  the  Sunda 
Strait,  between  Java  and  Sumatra.  On  May  20,  1883,  pre- 
monitory signs  of  the  great  eruption  which  followed  began 
to  be  observed.  There  were  sounds  like  artillery,  earth- 
quake shocks,  and  clouds  of  ash.  On  August  26th  to  28th 
terrific  explosions  took  place,  by  which  a  large  part  of  the 
mountain  was  destroyed.  Lamps  were  lighted  in  the  day- 
time in  surrounding  regions,  and  quantities  of  pumice  fell 
on  the  decks  of  such  ships  as  were  sailing  in  those  waters. 
So  unusual  was  the  display  of  fiery  energy  that  the  erup- 
tion has  been  made  the  subject  of  careful  investigation  and 
special  report,  both  by  the  Royal  Society  of  Great  Britain 
and  by  the  Dutch  Government.  According  to  Verbeek,  the 
author  of  the  latter  report,  the  fine  dust  rose  to  a  height  of 
50,000  feet.  Such  dust  fell  on  ships  1,600  miles  away,  and 
is  generally  believed  to  have  been  swept  around  the  world. 


152  GEOLOGY 

To  it  are  attributed  the  lurid  red  skies  which  were  observed 
in  America  during  the  months  of  the  following  autumn. 

Perhaps  the  most  interesting  results  of  the  eruption 
were  the  atmospheric  and  sound  waves  and  water  waves 
generated.  The  atmospheric  waves  were  seven  times  re- 
peated around  the  globe.  The  explosions  were  heard  to 
almost  incredible  distances,  as  at  Bangkok,  1,413  miles; 
Australia,  2,000  miles ;  Ceylon,  2,058  miles ;  and  at  the 
Chagos  Islands  in  the  Indian  Ocean,  2,267  miles.  It  was  as 
if  an  explosion  in  New  York  were  heard  in  the  Bermudas, 
Newfoundland,  Duluth,  New  Orleans,  and  Denver.  Water 
waves  are  believed  to  have  swept  over  half  the  globe.  It  is 
probably  not  too  much  to  say  that  this  single  eruption  af- 
fected the  entire  planet. 

THE  CAUSES  OF  VOLCANIC  ACTION 

128.  No  conclusive  word  has  been  spoken  on  this  sub- 
ject. A  final  conclusion  may  lie  beyond  the  bounds  of 
human  investigation.  As  with  the  glaciers  of  the  Ice  age, 
earthquakes,  or  the  origin  of  species,  many  facts  are  known 
and  some  major  conclusions  can  not  be  doubted,  but  the 
ultimate  cause  yet  lies  in  obscurity.  This  is  not  discour- 
aging to  the  true  student,  but  only  one  of  many  proofs  of 
the  limitation  of  our  knowledge. 

Two  questions  must  be  asked  :  (1)  What  are  the  sources 
of  the  heat  needed  to  melt  the  rocks  and  form  the  gaseous 
clouds  of  vast  extent  ?  (2)  What  is  the  force  that  expels 
these  materials  from  the  crust  ?  As  regards  the  first,  many 
answers  have  been  given  which  must  be  set  aside,  or  at 
least  appear  to  be  inadequate.  Volcanic  heat  has  been 
thought  to  be  a  remnant  of  the  original  heat  of  the  globe. 
But  on  this  theory  it  would  seem  that  vulcanism  should 
gradually  have  declined  through  the  ages  of  geological  his- 
tory. This  has  not  been  proved,  and  some  facts  look  in 
the  opposite  direction.  Others  have  thought  that  the  heat 
comes  from  powerful  chemical  reactions  in  the  deep  natural 


VOLCANOES  153 

laboratories  of  the  earth's  crust.  But  this  cause  seems  in- 
sufficient for  the  melting  that  is  accomplished.  It  is  known 
that  rocks  melt  and  become  plastic,  under  great  pressure, 
in  the  presence  of  water  and  alkaline  materials.  This  is 
called  aqueo-igneous  fusion,  but  does  not  account  for  the 
great  heat  of  volcanoes.  All  these  causes  may  contribute 
to  the  result  in  various  degrees  and  localities.  Thus  the 
original  heat  of  the  earth  is  believed  to  hold  the  rocks  of 
the  crust  near  the  melting  point  at  no  great  distance  below 
the  surface.  Such  crushing  as  goes  with  mountain-making 
may  add  heat  enough  of  mechanical  origin  to  cause  fusion. 
A  seemingly  paradoxical  theory  is  that  the  under  rocks 
would  melt  if  they  were  not  under  stupendous  pressure, 
and  when  at  some  points  this  pressure  is  relieved  in  the 
succession  of  strains  which  the  crust  undergoes,  the  rocks 
there  pass  into  a  molten  state.  The  two  theories  are  not 
inconsistent.  At  one  point  we  have  added  pressure  and 
added  heat ;  at  another,  removal  of  pressure  and  lowering 
of  the  melting  point. 

As  to  the  cause  of  explosion,  the  answer  is  hardly  more 
satisfactory.  The  sudden  conversion  of  water  into  steam  is 
thought  to  have  much  to  do  with  explosive  eruptions,  and 
the  more  as  most  volcanoes  are  near  the  sea.  But  this  does 
not  account  for  inland  or  quiet  eruptions.  Xor  is  it  easy 
to  see  how  enough  water  could  reach  the  seat  of  the  heat, 
either  by  saturation  or  sudden  inflow.  Chains  of  volcanoes, 
as  in  the  Andes,  correspond  with  lines  of  mountain-making. 
A  perfect  theory  must  explain  all  the  facts.  Why  are 
some  eruptions  quiet  and  others  violent  ?  Why  the  differ- 
ences in  the  composition  and  heat  of  lavas  ?  Why  should 
there  be  a  difference  of  10,000  feet  in  the  height  of  the  lava 
columns  in  the  adjacent  volcanoes  of  Mauna  Loa  and 
Kilauea?  Why  are  some  vents  intermittent  and  others 
constant  in  action  ?  Such  are  some  of  the  questions.  No 
theory  covers  all  of  these. 


CHAPTER  IX 

MOVEMENTS  OF  THE  EARTH'S  CRUST 

129.  THESE  may  be  roughly  subdivided  into  three  classes, 
which  are  treated  in  the  following  pages.     We  may  have 
movements  with  shock  or  earthquakes,  gentle  upward  and 
downward  movements  or  oscillations,  and  movements  pro- 
ducing folding  or  deformation.     The  last  two  are  imper- 
ceptible as  processes  by  the  ordinary  means  of  observation, 
but  may  at  intervals  be  accompanied  by  the  first. 

I.  EARTHQUAKES 

130.  As  with  volcanoes,  so  here  we  shall  find  it  profit- 
able to  begin  with  a  concrete  example.     We  choose  the 
Charleston  earthquake  of  1886. 

Slight  shocks  had  occurred  during  the  summer  months, 
and  the  great  shock  took  place  about  9.50  p.  M.,  August  31, 
1886.  It  lasted  sixty  to  seventy  seconds.  All  the  build- 
ings of  Charleston  were  affected,  and  many  seriously  in- 
jured. The  peculiar  wave  motion  of  an  earthquake  was 
powerful  and  destructive.  The  amplitude  of  the  wave,  or 
measure  of  sidewise  motion,  was  3  to  4  inches.  The  worst 
effects  were  produced  on  land  formed  by  reclaiming  parts  of 
the  bay.  The  waves  traveled  across  country  at  the  rate  of 
150  miles  per  minute.  The  epicentral  tracts — that  is,  the 
places  directly  over  the  centers  of  disturbance  (for  there 
were  two) — were  at  Woodstock,  16  miles  north-northwest  of 
the  city,  and  at  a  point  13  miles  west.  At  these  points  were 
evidences  of  vertical  movements,  such  as  sunken  piers  and 
154 


MOVEMENTS  OP  THE  EARTH'S  CRUST 


155 


joists  lifted  from  their  positions.     As  one  receded  from  the 

centers  evidences  of  lateral  thrust  were  found,  especially 

sharp    bends    in    the    railway 

tracks  passing  both  places.     At 

some  places  craterlets  opened, 

and  water  stained  with  earth 

was  belched  into  the  air. 

The  depth  of  the  focus  was 
determined  as  about  12  miles. 
The  sounds  were  described  by 
the  terrified  people  by  a  re- 
markable variety  of  illustra- 
tions— "  thunder  in  the  ground  " ;  "  roaring  of  a  prairie 
fire  " ;  "  troop  of  cavalry  crossing  a  long  bridge  "  ;  "  run- 
ning of  heavy  machinery  in  basement  of  houses  "  ;  "  train 
of  cars  at  a  distance " ;  "  escape  of  steam  from  a  boiler, ' 
and  many  others.  This  roar  is  ascribed  to  the  vibration 
and  clash  of  many  objects,  and  is  well  likened  to  the 
aggregate  of  sounds  in  a  great  city. 

A  wide  territory  felt  the  shock.     The  region  of  sensible 
shaking  is  given  as  a  circle  having  a  radius  of  1,000  miles, 


FIG.  75.— Section  of  pier  driven  down- 
ward by  earthquake,  Charleston, 


FIG.  76.— Bends  in  railway  track  near  Charleston,  made  by  earthquake,  1886. 

an  area  of  about  2,500,000  square  miles.  Shocks  exciting 
special  attention  were  felt  in  New  York,  in  Cleveland,  in 
the  province  of  Ontario,  and  in  the  Mississippi  Valley  as 
far  west  as  Iowa  and  Missouri.  Much  of  Pennsylvania  was 
a  region  of  "earthquake  shadow."  Valuable  determina- 
tions of  the  rate  of  transmission  of  the  shock  were  made 


156  GEOLOGY 

because  of  the  wide  use  of  standard  time,  which  made  pos- 
sible a  careful  comparison  of  the  time  when  the  disturbance 
was  felt  in  widely  remote  areas.  The  investigation,  pro- 
longed as  it  was,  gave  no  clew  to  the  ultimate  cause  of  the 
earthquake. 

Principles  of  Earthquake  Action 

131.  Definitions. — An  earthquake  is  a  trembling  or  shak- 
ing of  a  part  of  the  earth's  crust.  It  may  involve  simple 
wave  motion  or  it  may  produce  a  permanent  displacement. 
Seismic  (Greek,  shaking}  is  an  adjective  synonym.  Seis- 
mology is  the  science  of  earthquakes,  and  includes  an 
extensive  literature.  The  focus  is  the  subterranean  center 


FIG.  77.— Building  unaffected  because  resting  on  soft,  inelastic  strata,  c,  C. 

of  disturbance  or  the  point  where  the  energy  is  applied. 
The  epicentrum  is  the  point  on  the  surface  directly  over 
the  focus.  The  seismic  vertical  is  a  straight  line  joining 
the  focus  and  epicentrum.  Earthquakes  are  closely  imi- 
tated by  some  shocks  of  artificial  origin.  Thus  the  explo- 
sion of  Hell  Gate  for  deepening  the  passage  between  the 
East  River  and  Long  Island  Sound  started  waves  of  move- 
ment in  the  rocks  which  were  felt  in  Boston,  200  miles  dis- 
tant. Similar  vibrations  are  often  felt  from  blasting  in 
quarries,  the  firing  of  heavy  guns,  and  the  passage  of  cars 
over  hard  or  frozen  ground. 

132.  Earthquake  waves.— As  described  by  Le  Conte,  an 
earthquake  is  caused  by  the  arrival  at  any  given  point  of 


MOVEMENTS  OP  THE  EARTH'S  CRUST  157 

an  elastic  compression  wave.  Even  the  hardest  rock  is 
more  or  less  compressible,  and  force  applied  is  transmitted 
to  a  greater  or  less  distance,  according  to  its  original 
strength  and  the  elasticity  of  the  medium.  The  waves 


Fio.  78.— Diagrammatic  view  of  a  part  of  the  earth's  crust  shaken  by  earthquake, 
x,  focus  ;  a,  6,  c,  d,  sections  of  spherical  waves  ;  b",  c",  d",  perspective  of  sur- 
face wave.— After  LB  CONTK. 

that  pass  up  from  the  focus  emerge  at  right  angles  to  the 
surface,  and  cause  an  upthrow  of  surface  objects,  as  seen 
in  the  Charleston  earthquake.  Other  waves  emerge  at  less 
angles  until,  at  distant  points,  they  move  in  lines  nearly 
parallel  to  the  surface  (Fig.  78).  By  observing  the  rela- 
tions of  the  various  dislocations  and  fractures  it  is  often 
possible  to  determine  the  position  of  the  epicentrum  and 
the  depth  of  the  focus.  The  amplitude  of  a  wave  is  the 
amount  of  actual  movement  of  the  particles  affected.  This 
in  earthquakes  is  given  by  Dana  as  varying  from  less  than 
a  millimeter  to  possibly  a  foot. 

The  rate  of  movement  depends  on  the  character  of  the 
rocks.  If  they  be  spongy  and  soft,  the  rate  is  less,  as  also 
if  the  rocks  are  much  crushed  and  fractured.  It  is  also, 
however,  commonly  true,  as  in  the  Charleston  disturbance, 
that  buildings  suffer  more  upon  the  softer  rocks,  even 
though  the  wave  moves  more  slowly.  Slight  shocks  are 
apt  to  travel  more  rapidly,  perhaps  because  the  crushing  of 
the  rocks  is  in  less  degree.  Waves  have  been  felt  in  mines 
which  were  not  sensible  at  the  surface,  and  have  been 


15g  GEOLOGY 

imperceptible  in  one  place  while  evident  in  surrounding 
places.  All  such  facts  point  to  the  variable  capacity  of 
i  _  *  a  a  t,  _  5  c  rocks  for  transmit- 

--?^—  "V-\"1  ting  wave  motion. 
For  this  reason 
waves  do  not  spread 

'  to  equal  distances  in 

all  directions.  Thus 
the  area  affected  by 
a  shock  may  be  ellip- 


FG.  79.  -Emergence  of  earth  wave  from  A,  at  a,  b,  tical>  linear>  OI>  irreg- 

and  c,  may  produce  fractures  having  the  direc-  ular    in    f  Orm.        As 

tion  i-t,  3-U,  and  5-6.    From  such  cracks  the  ,.    ,  .  h 

position  of  focus  A  may  be  determined.  ngnt   WaV6S  maV    D 

reflected  or  refract- 

ed in  passing  from  one  medium  to  another,  so  may  earth- 
quake waves  in  passing  from  one  kind  of  rock  to  another. 

133.  Water  waves  due  to  earthquakes.  —  Such  are  often 
of    great  magnitude  and   destructive  power.      They  are 
"  forced  "  waves,  and  the  student  should  never  commit  the 
error  of  calling  them  tidal  waves.     They  sometimes  move 
at  the  rate  of  several  hundred  miles  per  hour.     Xot  seldom 
withdrawal  of  the  waters  from  the  shore  region  is  followed 
by  a  stupendous  onrush.     Such  waves  may  cross  the  widest 
seas.     During  the  Samoda  earthquake  in  Japan,  1854,  the 
bottom  of  a  bay  was  exposed,  where  commonly  there  were 
30  feet  of  water.     The  town  was  next  flooded  to  a  depth 
of  30  feet,  and  this  occurred  several  times.     Other  illus- 
trations will  be  given  in  the  account  of  particular  earth- 
quakes. 

134.  Distribution  of  earthquakes.  —  Such  movements  have 
been  common  throughout  the  geological  ages,  and  occur 
more  or  less  frequently  in  nearly  all  parts  of  the  earth. 
Like  glaciers  and  volcanoes,  therefore,  they  are  not  to  be 
thought  strange  or  peculiar,  but  are  a  normal  phase  of  the 
earth's  development.     They  occur  in  nearly  all  volcanic 
regions,  but  are  by  no  means  confined  to  these,  as  is  shown 


MOVEMENTS  OF  THE  EARTH'S  CRUST     159 

by  their  occurrence  in  the  eastern  and  central  parts  of  the 
United  States.  We  must  remember,  however,  that  historic 
records  in  America  are  brief.  Professor  Shaler  has  cited 
evidence  that  destructive  earthquakes  have  not  occurred 
in  New  England  since  the  glacial  times.  This  is  indicated, 
for  example,  by  the  perched  bowlders,  which  would  other- 
wise have  been  thrown  down  from  their  unstable  positions. 
The  presence  of  frail  rock  structures,  such  as  erosion  pil- 
lars, suggests  similar  freedom  from  agitation  for  other 
parts  of  the  country.  Still,  slight  shocks  are  not  seldom 
observed  in  the  eastern  United  States,  and  are  somewhat 
frequent  on  the  Pacific  coast.  The  western  border  of 
South  America  is  the  scene  of  most  frequent  and  destruc- 
tive shocks.  Earthquakes  are  not  uncommon  in  the  Alps, 
and  in  general  abound  in  the  region  of  the  Mediterranean, 
in  southern  Asia,  and  among  the  volcanic  islands  of  the 
Pacific.  They  have  been  observed  by  Milne  in  Japan, 
where  they  may  average  one  for  every  day  for  considerable 
intervals,  and  where  some  have  been  terribly  destructive. 
Slight  shocks  are  occasionally  felt  in  the  British  Islands, 
especially  in  Scotland.  Le  Conte  expresses ,  the  opinion 
that  at  all  times  some  part  of  the  earth  is  shaken  by  earth- 
quakes. 

135.  Geological  effects  of  earthquakes. — These  are  quite 
various,  but  only  occasionally  important.  Fragile  rock 
masses  may  be  broken  or  thrown  from  their  places,  and 
soils  or  masses  of  talus  jarred  down  the  slopes  on  which 
they  rest.  Planes  of  jointing,  cleavage,  and  bedding  may 
be  opened  to  air  and  water,  and  cavernous  masses  of  rock 
shaken  down.  Landslips  are  precipitated,  as  in  some 
instances  in  the  Alps.  Fissures  may  be  formed  or  widened, 
facilitating  the  circulation  of  waters,  the  making  of  veins, 
and  metamorphic  changes  in  the  rocks.  Some  permanent 
changes  of  level  have  resulted.  Possibly  the  greatest  effect 
of  earthquakes  is  the  destruction  of  colonies  of  marine 
creatures,  either  by  the  shock  or  by  the  agitation  of  the 


160  GEOLOGY 

fine  sediments  which  would  be  fatal  to  some  forms  of  life 
which  nourish  in  clear  waters. 

136.  Construction  of  buildings. — It   is  obvious   that  in 
regions  subject  to  shocks,  buildings  should  be  of  moderate 
height  and  solid  construction,  either  of  wood  or  of  thick 
walls  of  good  material,  well  bound,  and  upon  the  best  foun- 
dations available.     It  is  not  impossible  that  disaster  may 
thus  befall  the  occupants  of  the  tall  buildings  of  modem 
American  cities.     Even  the  waves  of  the  Charleston  earth- 
quake were  felt  to  an  unpleasant  degree  on  the  upper  floors 
of  the  Herald  Building  in  New  York. 

137.  Special  illustrations. — We  have  already  given  an  ac- 
count of  the  Charleston   earthquake.     Seventy-five  years 
earlier,  in  1811,  a  noteworthy  series  of  shocks  took  place  in 
the  vicinity  of  New  Madrid  on  the  Mississippi  River  in 
Missouri.     This  is  also  a  great  distance  from  any  volcanic 
region.     Powerful  ground  waves  moved  across  the  country, 
swaying  the  forests  and  often  causing  the  tree  tops  to  inter- 
lock.    The  ground  was  cleft  with  fissures,  which  ran  in  a 
fixed  direction,  insomuch  that  the  inhabitants  felled  trees 
in  a  line  at  right  angles  to  that  of  the  fissures  in  order  to 
find  safe  refuge  upon  them.     Some  areas  sank  so  that  trees 
were  left  standing  in  water,  and  new  lakes  and  islands  were 
formed  along  the  Mississippi  River.     Boundaries  of  prop- 
erty were  thrown  into  confusion,  and  the  Government  is 
said  to  have  made  a  reissue   of  lands  to  the  extent  of 
1,000,000  acres.     The  whole  district   is  one   of  extensive 
river  deposits,  and  this  may  account  for  the  frequency  of 
the  fissures  and  the  sunken  areas. 

In  the  following  year  a  most  disastrous  earthquake  vis- 
ited Caracas  in  South  America.  The  ground  undulated 
like  boiling  waters,  and  the  city  was  instantly  destroyed, 
with  the  loss  of  10,000  lives. 

One  of  many  other  severe  South  American  disturbances 
occurred  in  the  region  of  Chili  on  February  20,  1835. 
The  agitation  had  a  north  and  south  range  of  1,000  miles, 


MOVEMENTS  OF  THE  EARTH'S  CRUST 


161 


and  was  felt  for  500  miles  in  an  east  and  west  direction. 
The  retreat  of  the  waters  was  so  great  that  vessels  in  seven 
fathoms  were  grounded,  while, 
by  the  onrush  of  the  great  forced 
wave,  a  man-of-war  was  driven 
over  houses  on  the  shore.  A 
submarine  volcano  opened  in  67 
fathoms  of  water,  and  the  land 
was  rent  by  fissures.  The  most 
important  geological  result  was 
that  the  area  about  Concepcion 
was  permanently  raised  several 
feet,  so  that  the  marine  organ- 
isms which  had  lived  near  shore 
were  left  to  perish.  A  similar 
uplift  had  occurred 
Chili  in  1822,  and  it 
has  been  computed 
that  57  cubic  miles 
of  matter  were  lifted 
from  below  sea  level 
to  a  position  above  it. 
This  equals  the  amount  of 
solids  carried  out  by  the  Ganges 
in  four  centuries.  This  is  an  in- 
structive comparison  of  uplift- 
ing and  down -wearing  opera- 
tions. A  catastrophe  took  out 
of  the  sea  as  much  rock  as  a 
great  river  could  carry  back  in 
400  years.  Lyell  supposes  the 
focus  to  have  been  10  miles  deep, 
and  on  this  basis  concludes  that 
in  the  100,000  square  miles  af- 
fected 1,000,000  cubic  miles  of  rock  were  lifted.  Such  \ 
displays  of  force  are  quite  beyond  our  understanding, 


162  GEOLOGY 

but  have  been  of  frequent  occurrence  in  the  history  of 
our  globe. 

Our  next  example  is  a  New  Zealand  earthquake  of  1855. 
It  is  a  region  of  earthquake  disturbances.  In  the  year  1882, 
for  example,  28  shocks  are  reported,  1  severe,  10  "  smart," 
and  17  slight.  That  of  1855  was  forcibly  felt  about  Cook's 
Strait,  which  separates  the  two  greater  islands,  and  affected 
360,000  square  miles.  On  the  south  shore  of  the  northern 
island  a  band  of  lowly  marine  organisms  called  nullipores 
incrusted  the  rocks.  After  the  shock  these  were  found  at  a 
height  of  9  feet  above  the  tide.  The  beach  also,  at  the  foot 
of  the  cliff,  had  afforded  a  passage  for  cattle  only  at  low 
tide,  but  was  now  passable  at  all  times.  This  uplift  was 
traced  inland  for  90  miles.  Across  Cook's  Strait  the  move- 
ment was  one  of  depression.  Ships  had  been  accustomed 
to  sail  up  the  Wairau  River  for  supplies  of  fresh  water. 
They  were  obliged  to  go  3  miles  farther  to  pass  beyond  the 
reach  of  marine  water  after  the  earthquake.  It  must  not, 
however,  be  thought  that  these  sudden  uplifts  are  the  chief 
way  in  which  the  lands  have  risen  above  the  sea.  We  shall 
later  see  how  this  is  usually  done  without  shock  or  obser- 
vation. 

The  greatest  European  earthquake  of  modern  times 
took  place  at  Lisbon  in  1755 ;  60,000  people  perished  within 
six  minutes.  A  marble  quay  on  which  a  throng  had  gath- 
ered to  escape  from  falling  buildings  sank  beneath  100 
fathoms  of  water.  Neighboring  mountain  sides  were  shat- 
tered, and  the  sea  wave  was  very  powerful.  A  concussion 
was  felt  at  sea,  and  at  the  sea  border  there  were  recession 
and  onrush.  The  wave  was  60  feet  high  at  Cadiz,  and  15 
feet  off  the  Madeira  Islands.  It  was  strongly  felt  in  the 
harbor  of  Kinsale,  Ireland,  where  ships  were  whirled  about 
and  the  market  place  flooded.  The  tremor  was  detected  in 
Sweden,  in  the  Alps,  in  North  Africa,  and  on  the  Great 
Lakes  of  North  America. 


MOVEMENTS  OF  THE  EARTH'S  CRUST     163 

Causes  of  Earthquakes 

138.  There  are  doubtless  several  ways  in  which  energy 
can  be  suddenly  applied  to  the  rocks.     Many  shocks  result 
from  volcanic  explosions,  and  are  really  therefore  products 
of  volcanic  action,  and  due  to  the  same  causes.     But  the 
greater  number  of  earthquakes  are  probably  due  to  the 
shrinking  of  the  earth's  crust  in  cooling.     This  is  not  the 
place  for  more  than  a  notice  of  this  important  subject.     It 
must  suffice  here  to  observe  that  a  comparatively  stiff  crust 
over  a  cooling  and  shrinking  interior  must  also  shrink.     It 
becomes  cracked  in  some  places  and  folded  in  others.    Stu- 
pendous strains  are  thus  set  up  and  rock  masses  are  liable 
to  move  suddenly  upon  each  other.    Kock  walls  or  beds  are 
held  against  each  other  by  inconceivable  pressures,  but  even 
such  friction  may  be  overcome  and  sudden  slips  occur.    All 
points  to  which  the  impulse  is  transmitted  experience  an 
earthquake. 

It  has  been  found  that  earthquakes  occur  in  some 
regions  more  often  in  winter  than  in  summer,  and  that 
they  are  more  frequent  in  certain  phases  of  the  moon.  It 
thus  is  plain  that  the  balance  of  forces  is  delicate,  and  may 
be  broken  by  change  of  atmospheric  pressure  and  by  the 
attraction  of  outside  masses  of  matter. 

If  the  shrinking  of  the  earth's  outer  crust  of  cooler 
rocks  be,  as  above  supposed,  the  greatest  cause  of  earth- 
quakes, they  are  the  symptoms  or  incidental  accompani- 
ments of  the  quiet  but  more  stupendous  movements  which 
are  next  to  be  considered. 

II.  OSCILLATIONS 

139.  This  term  is  applied  to  gentle  and  long-continued 
movements,  by  which  parts  of  the  earth's  crust  rise  or  sink. 
The  amount  of  elevation  or  subsidence  is  measured  with 
reference  to  sea  level,  which  is  generally  believed  to  be 
practically  stable.     Such  movements  are  so  slow  that  they 


164:  GEOLOGY 

can  be  detected  only  by  careful  observers,  and  by  compari- 
sons made  at  long  intervals.  And  yet  they  are  among  the 
most  important  changes  which  affect  the  globe.  Mainly 
by  them  in  all  ages  have  the  sea  floors  risen  out  of  the 
water  to  form  existing  continents.  By  them  lands  of 
unknown  extent  have  been  buried  beneath  the  sea. 

140.  Proofs  of  elevation. — The  proof  that  elevations  of 
large  areas  have  taken  place  in  distant  periods  lies  not  only 
in  the  presence  of  marine  fossils  in  rocks  hundreds  or  thou- 
sands of  feet  above  the  sea,  but  also  in  the  fact  that  such 
elevations  are  now  in  progress,  and  have  been  in  operation 
in  historic  and  immediately  prehistoric  times.  If  barna- 
cles, which  attach  themselves  permanently  to  objects  with- 
in reach  of  the  tide,  are  found  above  high-tide  limit,  we 
have  a  proof  of  recent  elevation.  The  same  is  true  of  bor- 
ings of  marine  mollusks  or  other  evidence  of  the  presence 
of  marine  organisms  above  where  they  could  now  live.  We 
must  make  sure,  however,  that  such  remains  have  not  been 
moved  from  their  original  relative  positions. 

Products  of  erosion  above  tide,  or  above  storm  levels, 
have  the  same  value.  Such  are  sea  caves  and  raised  beaches, 
whether  they  are  notches  cut,  or  embankments  built,  on 
hill  or  mountain  slopes,  where  no  waves  can  now  work. 
Such  beaches  exist  about  the  basins  of  ancient  lakes  which 
have  been  destroyed  or  made  small  by  evaporation  or  drain- 
age. These  are  to  be  distinguished  from  ancient  sea  beaches, 
which  can  only  be  due  to  rising  of  the  land.  Such  beaches 
are  wide  and  strong  in  proportion  to  the  time  during  which 
the  land  remained  at  that  level.  A  succession  of  such 
beaches  on  a  slope  above  the  sea  means  that  there  were 
successive  periods  of  elevation,  alternating  with  periods 
of  stability.  Works  of  man,  such  as  piers,  are  now  some- 
times found  elevated  and  removed  from  the  sea.  Dis- 
tance alone  can  not  be  taken  as  proof  of  elevation,  because 
the  sea  may  be  crowded  off  by  filling,  as  in  so  many 
delta  regions.  Storm  beaches  also  might  be  taken  as 


MOVEMENTS  OF  THE  EARTH'S  CRUST  165 

proofs  of  elevation  by  those  who  are  strangers  to  the  power 
of  the  sea. 

141.  Regions  of  present  or  recent  elevation.— Since  the 
glacial  time  the  coast  of  Maine  has  risen  at  some  points 
somewhat  more  than  200  feet.     Similarly  many  points  to 
the  north,  as  Labrador   and   Newfoundland,  have  risen, 
though  some  points  along  intermediate  coasts  are  sinking. 
Terraces  of  recent  coral  limestone  are  found  on  the  shores 
of  Cuba  several  hundred  feet  above  the  sea  level.    Ele- 
vated beaches  are  common  about  Great  Britain.     Along 
the  coast  and  among  the  islands  of  western  Scotland  one 
may  follow  the  25-foot  beach  for  long  distances.     Scan- 
dinavia is  a  well-known  illustration.     Its  shore  line  is 
slowly  rising  except  south  of  Stockholm,  where  a  slight 
subsidence  is  in  progress.     The  uprising  increases  going 
northward  to  a  maximum  of  six  feet  per  century,  accord- 
ing to  Baron  de  Geer.     Sea  terraces  or  old  beaches  are 
found  far  inland  and  up  to  a  height  of  600  feet. 

There  is  no  clearer  or  more  interesting  example  than 
that  afforded  by  the  three  remaining  columns  of  the  Tem- 
ple of  Jupiter  Serapis  at  Puzzuoli,  near  Naples.  These 
stand  at  the  water's  edge,  and  were  erected  at  the  begin- 
ning of  the  Christian  era.  At  about  20  feet  up  they  bear 
a  broad  belt  of  perforations  made  by  boring  sea  mollusks. 
These  prove  that  the  place  has  sunk  20  feet  since  the  tem- 
ple was  built,  that  it  remained  under  water  long  enough 
for  the  borings  to  be  made,  and  that  it  has  now  risen  to 
about  its  original  relation  to  the  sea.  Such  an  oscillation 
in  a  volcanic  region  may  affect  less  territory  than  similar 
movements  on  the  east  coast  of  America  or  in  the  north  of 
Europe. 

142.  Proofs  of  subsidence. — Forests  or  stumps  standing  as 
they  grew  and  now  submerged  in  sea  water  are  proofs  of 
subsidence.     Precisely  the  same  is   shown  by  submerged 
beds  of  peat.     As  evidences  of  elevation  are  destroyed  by 
erosion,  so  these  proofs  of  depression  are  easily  buried  by 

13 


166  GEOLOGY 

sediments.  But  there  are  other  evidences  of  depression 
less  easily  destroyed.  Thus  the  animal  or  plant  forms  on 
two  sides  of  a  water  passage  may  be  so  similar  that  both 
groups  must  have  had  a  common  beginning,  and  have  been 
cut  apart  by  the  sinking  of  the  intervening  land.  Such  is 
the  relation  between  the  organisms  of  some  East  Indian 
islands  and  Australia. 

Another  and  most  important  proof  of  former  subsidence 
is  the  series  of  fiords  or  "drowned"  valleys  which  char- 
acterize the  eastern  coast  of  North  America,  the  shores  of 
Scotland  and  of  Norway.  The  inlets  of  the  Hudson  or 
Narragansett  and  of  the  Saguenay,  or  of  the  firths  of  the 
Forth  and  the  Clyde,  are  typical  examples.  The  sea  can 
not  cut  out  such  valleys  extending  far  inland.  They  were 
formed  by  the  common  means  of  valley-making  when  the 
land  was  higher  than  now,  and  on  the  down-sinking  of  the 
land  the  lower  valleys  were  drowned,  resulting  in  estuaries 
like  the  Hudson,  bays  like  Narragansett,  and  sea  lochs  like 
those  of  Scotland. 

143.  Regions  of  subsidence. — About  Cape  Ann,  at  New- 
buryport,  and  on  Nantucket  Island  recent  subsidence  of  a 
few  feet  has  been  shown.  Likewise  the  geologists  of  New 
Jersey  believe  that  the  shore  line  gives  evidence  of  recent 
sinking  at  the  rate  of  two  feet  per  century.  They  cite  as 
evidence  meadow  turf  covered  with  some  feet  of  water,  the 
encroachment  of  water  upon  the  shell  heaps  of  prehistoric 
dwellers,  buried  corduroy  roads  of  early  settlers,  the  dying 
off  of  trees  at  the  seaward  edge  of  fields,  and  the  migration 
of  oysters  up  the  streams.  Southward  the  great  inlets  of 
the  Delaware  and  Chesapeake  Bays  are  true  fiords,  though 
the  bordering  lands  fail  of  the  mountainous  heights  of  the 
Hudson  and  of  Norway.  The  Chesapeake  and  its  tidal 
rivers  occupy  just  such  a  group  of  valleys  as  would  be 
made  by  an  ordinary  river  system.  Such  is  their  origin, 
and  submergence  has  caused  the  present  conditions.  For- 
est remains  buried  several  hundred  feet  below  the  level  of 


MOVEMENTS  OF  THE  EARTH'S  CRUST     167 

the  Gulf  of  Mexico  at  J^ew  Orleans  prove  late  sinking  of 
the  Mississippi  Delta  district.  The  great  series  of  deep 
inlets  on  the  Pacific  coast  from  the  Golden  Gate  to  Alaska 
proves  subsidence.  As  this  is  written  there  appears  notice 
of  studies  undertaken  by  Mr.  G.  K.  Gilbert  to  determine 
what  the  present  movements  are  on  the  Pacific  coast. 

Six  hundred  miles  of  the  south  Greenland  coast  have 
been  sinking  during  recent  centuries.  Houses  once  high 
and  dry  are  now  washed  by  the  tide,  and  in  some  cases  low 
islands  have  been  covered,  leaving  the  remains  of  buildings 
standing  out  from  the  water.  So,  as  already  mentioned, 
the  southern  part  of  Sweden  has  recently  undergone  sub- 
sidence, though  the  present  continuance  of  this  is  ques- 
tioned. Past  submergence  is  proved  by  the  rising  of  sea 
waters  upon  parts  of  the  village  of  Scania.  We  here  find 
an  important  principle,  namely,  that  neighboring  places 
may  oscillate  in  reverse  directions.  Similarly  the  vast  sub- 
sidence which  Darwin's  theory  of  coral  islands  demands 
for  the  central  Pacific  is  believed  by  many  to  have  been 
accompanied  by  the  stupendous  mountainous  uplifts  of 
western  America.  Similarly  subsidence  has  been  shown 
for  the  coasts  of  Devon  and  Cornwall,  which  may  be  put 
over  against  the  late  uprisings  of  the  northern  shores  of 
Great  Britain.  The  earlier  history  of  the  Temple  of  Jupi- 
ter Serapis  shows  subsidence,  and  the  same  process  is  now 
reported  to  be  again  in  operation  there. 

144.  Warping. — It  is  now  well  for  us  to  try  to  picture 
the  character  of  movements  as  they  take  place  over  great 
regions.  So  long  as  the  crust  is  not  broken  the  uplift  or 
down  going  must  be  more  in  amount  in  some  places  than  in 
others.  It  culminates  at  one  point  or  along  an  axis,  and 
fades  out  in  all  directions.  An  uplift  is  therefore  a  very 
broad  swell  whose  height  is  relatively  insignificant.  If 
in  a  given  region  one  part  goes  up  or  down  faster  than 
another,  or  one  part  goes  up  and  another  goes  down,  such 
varying  movement  is  called  warping.  Gilbert  has  shown, 


168  GEOLOGY 

with  a  close  approach  to  demonstration,  that  such  a  warp- 
ing is  now  going  on  in  the  region  of  the  Great  Lakes.  The 
tilting  is  toward  the  southwest,  and  by  it,  if  it  continues 
for  a  few  thousand  years,  it  is  claimed  that  the  waters  of 
Niagara  will  be  turned  across  Illinois  into  the  Mississippi 
River.  The  rate  of  warping  is  5  inches  per  century  in  a 
distance  of  100  miles.  It  is  not  shown  whether  the  move- 
ment is  all  upward  at  varying  rates,  or  whether  this  great 
region  is  turning  as  upon  a  fulcrum. 

145.  Causes  of  oscillations. — Here  we   must  meet  the 
obscurity  which  attends  the  whole  problem  of  the  interior 
of  the  earth.     Physicists  and  geologists  of  the  present  day 
ascribe  these  changes  mainly  to  the  slow  shrinking  of  the 
earth.     But  it  is  not  denied  that  other  causes  may  con- 
tribute something  to  the  result.     Thus  (1)  sea  level  itself 
may  slightly  vary,  owing  to  the  attraction  of  great  conti- 
nental and  mountain  masses  or  extensive  ice  sheets.     (2) 
Heat  invading   masses  of  rock    from    below  may  cause 
expansion  and  uplift.     (3)  Loading  and  unloading,  as  by 
erosion  in  one  place  and  deposit  in  another,  may  affect  the 
stability  of  the  crust.     A  great  thickness  of  coarse  sand- 
stone, a  rock  which  could  only  be  formed  in  shallow  water, 
proves  subsidence,  which  may  be  due  to  loading.     In  like 
manner  the  gathering  and  melting  off  of  an  ice  sheet  over 
hundreds  of  thousands  of  square  miles  may  promote  oscil- 
lations.    But  none  of  these  causes  can  explain  the  first 
uprisings  of  land  from  primeval  waters. 

III.  MOVEMENTS  OF  DEFORMATION 

146.  Definition. — Such  movements  can  not  be  sharply 
marked  off  from  oscillations.     But  they  include  in  a  gen- 
eral way  all  changes  of  position  of  parts  of  the  earth's 
crust  by  which  considerable  folds  are  made  or  dislocations 
produced.     An  oscillation  is  a  slight  bend  up  or  down. 
By  folding,  arches  of  considerable  height  and  troughs  of 
large  depth  are  formed.     There  may  be  all  gradations  be- 


MOVEMENTS  OF  THE  EARTH'S  CRUST     169 

tween  the  two.  It  is  by  deformatory  movements  that  typ- 
ical mountain  chains  have  their  origin.  It  will  be  more 
convenient  to  study  mountain  structures  and  mountain- 
making  at  various  points  in  our  account  of  structural  and 
historical  geology.  The  subject  is  briefly  introduced  here 
because  the  forces  at  work  are  believed  to  be  the  same 
with  those  that  cause  oscillations  and  many  earthquakes. 

If  a  broad  upward  swell  takes  place  over  a  part  of  a 
continent,  we  have  an  oscillation.  If  there  be  a  zone  of 
weakness  and  the  energy  is  applied  long  enough  and  pow- 
erfully enough,  a  series  of  folds  and  breaks  will  be  caused, 
forming  mountains.  In  the  upswelling,  and  especially  in 
the  crumpling,  sudden  shoves  or  reliefs  will  cause  earth- 
quakes. All  the  movements  concerned,  save  at  such  times 
as  earthquakes  occur,  take  place  with  exceeding  slowness. 


CHAPTER  X 

GEOLOGICAL   WORK   OP   ORGANISMS 

147.  THE  most  stupendous  changes  are  produced  in  the 
form  and  structure  of  the  earth's  crust  by  great  rivers,  gla- 
ciers, volcanoes,  and  the  waves  of  the  sea.     But  we  have 
already  seen  that  equally  important  effects  are  due  to  the 
quiet  but  constant  activities  of  the  atmosphere.     In  like 
manner  living  creatures,  plants  and  animals,  work  silently, 
but  produce  results  which  perhaps  compare  in  significance 
those  of  any  other  geological  agent.     We  take  first — 

148.  The  work  of  plants.— Plants  cover  the  greater  part 
of  the  land.     The  exceptions  are  the  rocky  slopes  of  the 
highest  mountains,  regions  covered  perpetually  by  ice  and 
snow,  and  a  few  very  dry  districts.     In  the  last,  however, 
some  vegetation  modified  to  suit  its  special  environment 
always  flourishes.     Everywhere  the  mantle  of  vegetation  is 
nearly  unbroken,  whether  of  dense  forests,  shrubs,  or  herba- 
ceous plants.     Their  effects  on  the  earth's  surface  are  most 
varied ;  in  fact,  they  are  inextricably  woven  together.     For 
convenience  of  study,  however,  we  must  seek  to  analyze 
them  and  discuss  them  one  by  one.     The  geological  effi- 
ciency of  plants  is  at  once  protective,  destructive,  and  con- 
structive. 

149.  Protective  effects  of  plants.— Plants  serve  to  shield 
the  soil  from  erosion  by  streams,  rain,  and  wind.     They 
may  also  prevent  the  frosts  from  reaching  a  depth  which 
they  would  otherwise  attain.     Many  hill  slopes  have  re- 
mained without  much  change  since  the  close  of  glacial 

170 


GEOLOGICAL  WORK   OF   ORGANISMS  171 

times,  a  period  of  at  least  some  thousands  of  years.  This 
would  be  impossible  but  for  the  forest  cover,  and  the  net- 
work of  roots  which  keeps  the  water  from  gathering  into 
channels  of  erosion.  This  is  evident  from  the  fact  that 
such  slopes  are  often  deeply  gullied  by  wet-weather  tor- 
rents within  a  few  decades  of  years  after  the  forests  are 
cut  away.  Roots  and  fallen  leaves  and  the  entire  body  of 
living  and  decomposing  vegetation  serve  as  an  absorbent 
mass  to  hold  the  water,  to  moisten  the  soil  for  further 
growth  of  plants  and  to  avert  floods. 

Trees  often,  though  not  always,  as  we  have  seen,  avail 
to  prevent  or  retard  the  descent  of  avalanches.  We  have 
also  observed  their  value,  whether  propagated  naturally  or 
by  the  hand  of  man,  in  checking  the  migration  of  dune 
sands.  They  also  serve  as  windbreaks,  as  in  Nebraska 
and  elsewhere  on  the  plains  of  the  West. 

150.  Destructive  effects  of  plants. — Wherever  the  roots  of 
trees  reach  down  to  the  rock,  they  insinuate  themselves  into 
its  crevices,  and  as  they  grow  they  help  to  rend  the  blocks 
apart.     Trees  often  grow  on  rocky  slopes  and  on  the  edge 
of  cliffs  where  there  is  little  soil,  and  the  roots  are  appar- 
ent as  they  rest  upon  and  descend  between  the  blocks  of 
rock.     It  is  most  common  to  see  slabs  partially  wedged  off 
and  nearly  ready  to  fall  to  the  bottom  of  the  cliff.     Roots 
of  trees  have  been  followed  to  a  distance  of  some  scores  of 
feet  from  the  parent  stock.     Thus  not  only  do  the  large 
roots  rend  the  rocks  and  heave  the  soil,  but  a  great  body 
of  rootlets  is  provided,  contact  with  which  promotes  dis- 
integration.     While  plants  withdraw  moisture  from  the 
soil,  they  also  may  mantle  the  earth  so  effectively  from  the 
direct  attack  of  the  sun  as  to  retard  evaporation,  and  pro- 
mote the  solvent   activity  which   water  always   exercises 
upon  rocks  and  soils. 

151.  Constructive  effects  of  plants. — These  are  chiefly  by 
way  of  accumulations  of  vegetable   remains.      The   most 
important  of  these  accumulations  is  the  mantle  of  decayed 


1T2 


GEOLOGY 


vegetation  which  everywhere  makes  up  a  part  of  the  true 
soils,  and  is  most  conspicuous  in  forests  and  the  deep  black 
soils  of  the  prairies.  We  also  find  important  accumula- 
tions in  special  situations.  Thus  on  the  shores  of  Florida 
flourishes  the  mangrove,  which  has  the  special  habit  of 
sending  out  branches  and  dropping  its  roots  through  the 


FIG.  81.— Rocks  wedged  apart  by  growing  tree,  western  Massachusetts. 

shallow  waters  of  the  sea  margin.  These  roots  entrap 
leaves,  waste  from  the  land,  and  remains  of  marine  organ- 
isms, and  thus  little  by  little  reclaim  areas  from  the  sea. 
Driftwood  may  accumulate  in  streams,  even  to  the  forma- 
tion of  a  solid  barrier  across  them.  Such  a  barrier  was  the 


GEOLOGICAL  WORK   OP  ORGANISMS  173 

great  raft  blockading  the  Red  River  in  Louisiana  for  a  dis- 
tance of  several  miles,  until  it  was  cut  away.  Such  block- 
ade would  retard  the  flow  of  the  stream,  hold  its  waters  to 
a  higher  level,  and  thus  change  the  geological  condition  of 
large  areas. 

Wherever  for  any  reason  drainage  is  imperfect,  vegeta- 
tion of  water-loving  types  flourishes,  and  we  find  the  deep 
black  earth,  and  in  favorable  conditions  the  peat  of  swamps. 
Thus  swamps  occur  on  the  flood  grounds  of  great  rivers,  in 
tracts  left  without  free  outflow  by  accumulations  of  gla- 
cial times,  in  the  inequalities  of  land  surfaces  recently 
elevated  out  of  the  sea,  on  the  sea  border,  and  even  on 
hill  and  mountain  slopes  of  considerable  inclination,  if 
the  climate  be  cool  and  the  supply  of  spring  water 
abundant.  In  the  shallow  waters  of  lake  borders  vege- 
table accumulations  are  important,  and  when  small  or 
shallow  lakes  become  nearly  filled  in  with  sediments  from 
the  land  the  whole  area  may  become  a  swamp  and  a 
field  of  organic  deposit.  Thus  in  a  general  way  much 
of  the  peat  of  to-day  and  the  coal  of  earlier  periods 
has  been  formed.  The  mode  of  origin  of  coal  will  be 
discussed  in  Part  III  (Chapter  XXI).  A  brief  account  will 
be  here  given  of  the  formation  of  peat.  Peat  is  partly 
decomposed  vegetable  matter.  It  is  formed  in  cool  cli- 
mates in  which  the  supply  of  moisture  is  plentiful  and 
usually  in  swampy  basins,  although,  as  indicated  above, 
deposits  of  peat  are  found  on  sloping  grounds.  Peat  beds 
may  be  many  feet  in  thickness,  and  the  vegetable  remains 
will  be  found  most  fully  disintegrated  at  the  lower  levels, 
while  near  the  surface  the  forms  of  twigs,  leaves,  and  mosses 
are  preserved  in  abundance.  Peat  is  chiefly  composed  of 
water-loving  plants,  particularly  of  the  genus  Sphagnum, 
but  may  contain  trunks  and  twigs  of  trees  which  grew  on 
the  spot  and  leaves  and  other  matter  drifted  in  by  currents 
of  air  or  water. 

Peaty  accumulations  may  extend  from  the  shores  over 


174  GEOLOGY 

the  waters  of  shallow  ponds,  thus  forming  quaking  bogs. 
Masses  of  peat  may  also  swell  during  rains  and  burst  and 
overflow  adjacent  territory  in  a  destructive  manner.  Not 
infrequently  a  deposit  of  peat  overlies  shell  marl.  The 


Fie.  82.— Growth  of  peat,  with  qnaking  bog.  A,  remnant  of  pond;  B,  B,  living 
Sphagnum ;  C,  C.  peaty  mass  from  disintegration  of  surface  layer  of  plants ; 
D,  D,  solid  part  of  swamp  with  trees.  -After  SHALER. 

marl  was  formed  during  the  period  of  lake  waters,  the  peat 
during  the  later  swampy  stage.  Peat  has  antiseptic  prop- 
erties, and  hence  the  bodies  of  men  and  animals  are 
sometimes  found  preserved  in  bogs.  Some  such  remains 
belong  to  prehistoric  times,  as  utensils  and  canoes  of  the 
lake  dwellers  and  skeletons  of  the  extinct  Irish  elk. 

Many  peat  beds  are  found  in  New  England,  including 
its  bordering  islands,  and  in  Canada.  In  Europe  they  are 
found  along  the  Loire  and  Somme  in  France,  in  Scandi- 
navia and  Scotland,  and  are  of  immense  extent  in  Ireland, 
where  from  one  tenth  to  one  seventh  of  the  country  is 
estimated  to  be  covered  by  them.  One  bog  is  reported 
to  have  an  extent  of  nearly  240,000  acres  and  a  depth  of 
25  feet. 

152.  Other  illustrations  of  the  geological  work  of  plants. — 
Minute  vegetable  organisms  called  diatoms,  living  in  lake 
and  ocean  waters  and  secreting  small  cases  or  shells  of 
silica,  are  sometimes  deposited  in  vast  numbers,  forming 
masses  of  so-called  diatomaceous  earth,  a  valuable  abrasive 
material.  Vegetable  acids  may  precipitate  silica  from  the 
water.  Thus  the  woody  fiber  of  some  ancient  trees  has 
been  replaced,  as  in  the  formation  of  the  beautiful  silicified 
woods  of  Arizona  and  South  Dakota.  Such  acids  also  have 


GEOLOGICAL  WORK  OF  ORGANISMS  175 

been  instrumental  in  the  accumulation  of  beds  of  iron  ore. 
The  iron  scattered  throughout  certain  rocks  in  minute  par- 
ticles is,  by  a  series  of  chemical  reactions,  dissolved  and 
brought  together.  Hence  it  is  that  coal  and  iron  are  some- 
times found  in  proximity,  as  in  Pennsylvania.  The  abun- 
dant vegetation  furnished  means  of  concentrating  the  iron, 
and  the  rocks  are  gray  and  comparatively  colorless  owing 
to  the  removal  of  the  iron,  to  which  rock  colors  are  largely 
due.  On  the  other  hand,  in  the  Connecticut  Valley,  where 
the  color  of  the  red  sandstones  is  due  to  the  disseminated 
iron  oxides,  no  beds  of  iron  ore  are  found. 

GEOLOGICAL  WORK  OF  MARINE  ANIMALS 

153.  Animals  that  live  in  water  contribute  to  the  his- 
tory of  the  earth,  chiefly  in  a  constructive  way,  by  the 
accumulation  of  their  remains,  often  in  masses  of  great 
extent  and  thickness.  An  exception  to  this  rule  is  found 
in  the  case  of  boring  mollusks,  which  perforate  wood  and 
even  rock  until  it  is  sometimes  honeycombed  and  de- 
stroyed. 

The  remains  of  sea  shells  and  other  hard  parts  of 
marine  organisms  accumulate  in  vast  quantities  on  the  sea 
bottom.  Thus  oysters  and  other  mollusks  occupy  a  tract 
of  sea  bottom,  live  and  die,  and  leave  their  shells  during  suc- 
cessive generations.  These  shells  consist  mainly  of  carbon- 
ate of  calcium  which  the  living  creature  has  secreted  from 
its  state  of  solution  in  sea  water.  If  the  water  be  shallow, 
as  close  to  the  shore,  the  shells  will  be  worn  and  broken  by 
the  waves.  With  the  entire  or  broken  shells,  and  with  fine 
mud  arising  from  their  grinding  by  the  waves,  is  mingled 
sediment  brought  to  the  sea  by  streams.  Thus  limestones 
are  formed,  of  great  extent  and  thickness.  For  example, 
there  are  three  great  Paleozoic  limestones  in  New  York, 
each  several  hundred  feet  thick  and  stretching  far  across 
the  State.  AYe  shall  have  further  occasion  to  notice  these 
organic  accumulations  in  our  study  of  rocks  and  when  we 


176 


GEOLOGY 


come  to  review  the  history  of  the  earth.  Suffice  it  here  to 
observe  that  most  limestones  of  the  earth's  crust,  includ- 
ing many  marbles  or  modified  limestones,  are  chiefly  due 
to  the  presence  in  the  sea  of  organisms  ranging  from  most 


FIG.  83.— Coral  limestone  bored  by  molluske. 

lowly  up  to  the  higher  species.  It  is  convenient  under  this 
head  to  refer  to  accumulations  made  by  mollusks  which 
inhabit  fresh  water.  They  are  not  geologically  important, 
at  least  as  regards  bulk,  but  are  common  and  of  consider- 
able interest. 

Eeference  has  been  made  to  beds  of  shell  marl  which 
often  underlie  peat.  In  many  lakes  or  ponds  countless 
small  mollusks  with  fragile  white  shells  of  calcium  car- 
bonate live  and  die,  and  their  shells  accumulate.  The 
shells  soon  break  up,  and  form  a  whitish  fine  ooze,  which 
when  dry  is  much  like  chalk.  A  rod  may  often  be  thrust 
many  feet  into  such  deposits  of  ooze,  which  have  been  gath- 
ering for  centuries.  It  is  such  bottoms  which  lead  to  the 
tradition  which  one  often  hears  in  the  country,  that  cer- 
tain small  lakes  are  bottomless.  In  time  the  water  becomes 
shallow,  vegetation  encroaches  upon  the  limits  of  the  pond 


GEOLOGICAL  WORK  OF  ORGANISMS  177 

as  already  described,  and  the  deposit  ceases  and  is  covered 
from  view. 

154.  Corals. — These  creatures  are  seldom  seen  by  dwell- 
ers in  the  cooler  zones,  except  as  specimens  in  museums. 
Little  but  the  hard  parts  of  coral  can  thus  be  preserved, 
and  hence  much  of  their  grace  of  form  and  most  of  their 
wonderful  coloring  are  lost.     Nevertheless,  they  have  long 
had  interest  for  students  of  natural  history  by  reason  of 
their  beauty,  their  remoteness  from  most  civilized  regions, 
and  their  importance  in  world-making.      Most  erroneous 
notions  have   often  been   popularly  held  about  them,  as 
that  they  were  "  insects  "  and  laid  the  foundations  of  their 
works  in  deepest  seas.     It  is  the  part  of  geological  study 
to  give  true  views  of  this  as  of  all  agents  which  modify  the 
crust  of  earth.    This  is  the  more  true  because  of  the  impor- 
tance of  coral  structures  and  accumulations  in  decipher- 
ing the  history  of  the  globe.     They  are  rightfully  regarded 
as  indices  of  a  warm  climate,  and  therefore  when  found  in 
the  rocks  of  temperate  or  polar  regions  are  demonstrative 
of  great  climatic  changes.     They  are  also  important  for 
the  bulk  of  their  contributions  to  the  rocks  of  the  earth's 
crust,  insomuch   that  ancient  formations  are   sometimes 
properly  referred  to  as  fossil  coral  reefs.     Thus  warm  seas 
may  be  proved  to  have  rolled  their  waves  where  now  is 
found  the   interior   of  a  continent.      Some   of  the  most 
famous  names  in  the  history  of  science  also  are  identified 
with  the  study  of  corals.     Here  we  think  of  Darwin  and 
the  voyage  of  the  Beagle,  Dana  and  the  Wilkes  Exploring 
Expedition,  of  the  elder  and  the  younger  Agassiz,  and  other 
naturalists. 

155.  Conditions  of  growth  of  corals.— Corals  are  marine 
animals,  and  can  not  flourish  in  waters  whose  temperature 
falls  below  68°  F.     The  waters  must  be  also  comparatively 
clear  and  free  from  the  muddy  sediment  which  rivers  bring 
from  the  lands.     The  species  which  are  of  importance  in 
rock-making  live  within  15  or  20  fathoms  of  the  surface. 


178 


GEOLOGY 


This  at  once  disposes  of  the  fancy  that  they  lay  the 
foundations  of  islands  in  the  abyssal  depths  of  the  ocean. 
Corals  must  also  be  exposed  to  the  open  surf ;  they  do  not 
flourish  in  quiet  and  protected  waters. 


FIG.  84.— Patch  of  corals  on 


Saville  Kent.) 


156.  Distribution  of  corals  in  existing  seas.— It  will  be 
seen  from  the  temperature  limit  as  given  above  that  corals 
can  only  thrive  in  tropical  or  subtropical  seas.  Thus  they 
are  widely  distributed  in  the  Pacific  Ocean,  in  the  torrid 
zone,  and  extend  in  some  cases  a  few  hundred  miles  beyond 
the  tropics.  Coral  islands  are  low,  and  are  thus  distin- 
guished from  the  high  or  volcanic  islands  of  the  ocean. 
They  are  very  abundant— nearly  three  hundred  in  num- 
ber, according  to  Dana,  besides  many  reefs  about  other  is- 
lands. Corals  are  not  abundant  on  the  American  side  of  the 
Pacific,  owing  to  the  currents  of  colder  waters  which  prevail. 


GEOLOGICAL  WORK  OF  ORGANISMS  179 

Coral  formations  are  found  as  reefs  near  certain  islands  of 
the  Hawaiian  group  as  far  north  as  28°  39'.  Their  limits 
south  of  the  equator  are  narrower.  Important  coral 
masses  are  found  in  the  Indian  Ocean  and  up  to  30°  north 
latitude  in  the  Red  Sea,  whose  waters  are  very  warm. 
Abundant  reefs  occur  in  the  West  Indies  and  off  the  coast 
of  Florida.  According  to  Le  Conte,  Key  West  owes  its 
existence  to  the  business  of  wreckage  incident  to  the 
perils  of  navigation  due  to  coral  reefs.  The  Bermudas 
are  the  most  northerly  group  of  coral  islands,  reaching 
to  32°  15'. 

157.  Coral  reefs  and  coral  rocks. — The  coral  reef  con- 
sists of  a  mass  of  coral  debris  which  nearly  or  quite  attains 
the  surface  of  the  sea.  Part  of  the  so-called  reef  is  thus 
submerged,  and  bears  a  forest  or  garden  of  living  corals 
whose  petaloid  forms  and  brilliant  colors  suggest  a  lux- 
uriant growth  of  flowers.  Other  areas  are  raised  above  the 
surface  of  the  sea  as  islands.  The  corals  are  often  broken 
up  by  the  waves,  and  so  piled  by  the  surf  as  to  stand  out 
of  the  water  precisely  as  the  fragments  of  common  rocks 
may  do.  Such  sands  may  then  be  attacked  by  the  winds 
and  built  into  still  higher  structures,  as  we  have  already 
learned  in  the  case  of  the  Bermudas.  We  may  now  look 
more  closely  at  the  origin  of  coral  rocks.  They  are  lime- 
stones, because  the  hard  parts  of  the  reef-building  corals 
are  composed  of  carbonate  of  calcium.  The  corals  grow  in 
a  variety  of  forms,  some  hemispherical  and  massive,  others 
branching  and  treelike,  and  often  most  delicate  and  fragile. 
The  latter  forms  are  readily  broken  by  the  waves,  and  the 
resistant  masses  are  buried  by  the  fragments  of  the  others, 
by  the  broken  shells  of  mollusks,  and  by  the  mud  produced 
by  the  grinding  of  the  surf  mill  or  drifted  from  neighbor- 
ing shores.  Thus  coral  sands,  coral  in  place,  and  miscella- 
neous materials  unite  and  are  at  length  consolidated  in 
ways  to  be  hereafter  explained,  and  we  may  find  a  compact 
rock  which  rings  under  the  hammer  and  from  which,  by 


180 


GEOLOGY 


subsequent  interior  changes,  much  of  the  coral  structure 
may  have  been  lost. 

158.  Kinds  of  coral  reefs. — They  are  of  three  sorts:  (1) 
Fringing  reefs.  Along  the  shore  of  any  island  or  other 
land  where  conditions  favor,  corals  may  grow.  Thus  a 


FIG.  85. — Section  of  island  with  fringing  reef,  a,  a. 

fringing  reef  is  formed  whose  inner  margin  is  composed  of 
a  belt  of  materials  which  have  become  subaerial  through 
wind  and  waves.  Its  outer  margin  consists  of  submerged 
coral  rock  and  living  corals,  extending  down  to  the  limit 
of  depth.  The  land  thus  bordered  may  be  of  volcanic  or 
other  origin.  (2)  Barrier  reefs.  These  lie  at  a  greater  or 
less  distance  from  a  shore,  with  which  they  are  roughly 
parallel.  The  nearly  inclosed  areas  of  protected  water  are 
called  lagoons.  The  reefs  are  often  interrupted  opposite 
streams,  whose  earthy  load  and  fresh  water  are  unfavorable 
to  the  corals.  The  great  barrier  reef  of  the  east  coast  of 
Australia  is  1,250  miles  long,  and  is  from  10  to  90  miles 
away  from  the  mainland.  (3)  Atolls.  An  atoll  is  an  elon- 
gated or  irregular  belt  of  low  coral  islands  nearly  inclosing 


FIG.  86. — Section  of  island  with  lagoons  and  barrier  reefs.    Slopes  much 
exaggerated. 

a  shallow  central  lagoon.  The  islands  may  be  two  or  three 
or  many  in  number,  with  a  corresponding  number  of  water 
channels  leading  from  the  lagoon  into  the  open  sea.  Corals 


GEOLOGICAL  WORK  OF  ORGANISMS 


181 


flourish  but  poorly  in  the  quiet  interior  waters,  but  grow 
chiefly  on  the  outside,  where,  below  their  limit,  the  sea  bot- 
toms may  descend  to  profound  depths. 

159.  Origins  of  barriers  and  atolls. — It  was  formerly  sup- 
posed that  atolls  were  the  coral-covered  rims  of  submerged 
volcanic  craters.  While  volcanic  islands  are  numerous,  it 
is  incredible  that  the  summits  of  several  hundred  sub- 


FIG.  87.— Section  of  atoll  with  central  lagoon. 

merged  cones  should  be  at  the  right  horizon  to  support 
coral  growth.  Darwin  propounded  the  subsidence  theory, 
which  was  generally  accepted,  and  still  holds  an  important 
but  disputed  place.  A  brief  statement  is  as  follows  :  If  a 
fringing  reef  borders  an  island  which  rests  on  a  sinking  sea 


FIG.  88.— Diagram  illustrating  the  subsidence  theory  of  coral  reefs.  I",  I'  and  I 
represent  sea  level,  which,  for  convenience,  is  shown  as  rising. 

bottom,  the  growth  of  the  coral  mass  upward  may  keep 
pace  with  the  subsidence.  As  the  corals  flourish  best  in 
the  outer  surf,  the  accumulation  will  grow  upward  at  that 
point,  leaving  a  depression  and  growing  lagoon  behind.  If 
the  subsidence  continues  the  original  island  will  disappear, 
and  we  shall  have  a  body  of  water  nearly  inclosed  by  a  reef, 
13 


182  GEOLOGY 

or  an  atoll.  All  stages  of  this  progress,  according  to  Dar- 
win and  Dana,  are  seen  among  the  coral  islands  and  reefs 
of  the  Pacific  Ocean.  They  thus  regard  as  proved  the. sub- 
sidence of  vast  central  Pacific  areas  during  a  long  period  of 
time.  Agassiz,  Murray,  and  others  have,  however,  shown 
that  some  barriers  and  atolls  have  been  formed  without 
subsidence,  particularly  in  the  West  Indies.  The  founda- 
tion on  which  the  corals  work  must  be  within  150  feet  of 
the  surface,  since  the  corals  only  thrive  in  comparatively 
shallow  water.  It  is  claimed,  and  with  truth,  that  volcanic 
cones  may  be  cut  off  by  the  waves  just  below  sea  level,  and 
it  has  also  been  shown  that  in  some  cases  shoals  of  sedi- 
mentary origin  have  been  occupied  by  corals.  It  is  claimed 
that  the  lagoons  associated  with  atolls  and  barrier  reefs, 
which  are  so  readily  accounted  for  by  the  subsidence  the- 
ory, may  also  be  explained  in  another  way.  Behind  the 
belt  exposed  to  the  surf  the  corals  do  not  flourish,  and  such 
coral  structures  as  are  formed  in  these  more  protected 
waters  are,  it  is  held,  gradually  dissolved  and  the  carbonate 
of  lime  swept  out,  leaving  basins  of  quiet  water.  Those 
who  desire  a  fuller  account  of  the  several  theories  are  re- 
ferred to  larger  text-books,  and  to  the  special  works  of 
the  authors  cited  above.  A  brief  statement  is  at  least  use- 
ful in  showing  that  there  are  important  problems  in  the 
science  of  the  earth  which  have  been  but  partly,  if  in  any 
measure,  solved.  We  know  many  facts  and  some  laws  of 
Nature.  That  the  deepest  and  largest  questions  await 
their  answer  is  not  a  misfortune,  but  a  high  incentive. 

160.  Age  and  rate  of  growth  of  corals. — Few  facts  of  ob- 
servation are  available.  We  know,  however,  that  the 
growth  is  slow,  and  that  any  considerable  coral  accumula- 
tions must  far  outrun  historic  time.  One  authority,  refer- 
ring to  coral  communities  of  6  to  9  feet  diameter  in  the 
Red  Sea,  thinks  that  they  might  have  been  seen  by  the 
Pharaohs.  At  least  corals  illustrate  the  value  of  causes 
which  operate  silently  throughout  long  periods,  and  they 


GEOLOGICAL  WORK  OF  ORGANISMS  183 

also  show  in  a  remarkable  degree  the  ability  of  frail  organ- 
isms to  flourish  under,  and  indeed  by  means  of,  the  power- 
ful attack  of  the  waves. 

161.  How  a  reef  becomes  inhabited. — After  an  island  is 
thus  formed  by  organic  and  physical  agencies,  it  may  receive 
the  germs  of  land  life  in  a  variety  of  ways.     Seeds  of  trees 
and  other  plants  may  be  drifted  upon  the  waters,  or  blown 
by  the  winds  from  neighboring  lands.     Others  may  be  car- 
ried in  the  crops  of  birds,  and  in  later  ages  life  of  various 
kinds  has  migrated  by  the  hand  of  man. 

GEOLOGICAL  WORK  OF  LAND  ANIMALS 

162.  Burrowing  animals. — Such  are  some  species  of  mole, 
the  muskrat,  woodchuck,  and  prairie  dog  among  the  verte- 
brates.    Their  borings  and  tunnelings  stir  the  soil,  and 
particularly  are  effective  in  bringing  up  the  subsoils  to  a 
horizon  at  which  they  may  be  made  suitable  for  vegetable 
growth.     The  geologist  is  not  infrequently  aided  by  these 
creatures,  especially  in  his  study  of  the   drift  deposits. 
The  crayfish  also,  of  the  class  of  crustaceans,  sometimes 
perforates  the  levees  of  the  Mississippi  to  such  an  extent 
as  to  cause  a  break  in  the  barrier  and  flooding  of  the  low- 
lands.    The  most  widespread  and  important  operations  are 
carried  on  by  the  common  earthworm,  as  shown  by  Dar- 
win's observations.     Soil  and  drift  to  the  depth  of  several 
feet  are  perforated  by  these  animals,  which  pass  much  of 
the   material   through  their  digestive  tracts,  where  it  is 
ground  or  dissolved  and  fitted  thus  for  the  use  of  plants. 
No  inconsiderable  quantity  of  such  earth  is  cast  by  them 
upon  the  surface,  so  that  Darwin  estimates  a  gain  of  1  to 
1|  inches  over  the  general  surface  in  ten  years.     To  this 
cause  he  attributes  the  disappearance  by  burial  of  stones  in 
fields,  and  the  covering  of  old  foundations  and  pavements. 
All  such  subsoiling  and  transport  to  the  surface  favor  gen- 
eral erosion  by  rains  and  winds,  and  are  of  course  important 
in  an  agricultural  way.     The  worms  are  larger  and  do  more 


184:  GEOLOGY 

work  in  some  moist  tropical  regions,  where  lawns  are  some- 
times rolled  to  crush  the  worm  castings. 

163.  Beavers. — The  beavers  build  dams  often  several 
feet  high  and  many  rods  in  length,  and  thus  in  some  cases 
flood  hundreds  of  acres  of  low-lying  land,  or  cause  it  to  be- 


FIG.  89.— Aspens  felled  by  beavers,  Colorado. 

come  marshy.  They  also  may  interrupt  drainage  by  felling 
trees  thickly  over  an  area.  Thus  vegetation  of  an  aquatic 
sort  is  favored,  and  beds  of  peat  may  be  formed.  A  history 
of  Orleans  County,  New  York,  records  that  flooding  by 
beavers  was  extensive  there  in  the  time  of  the  pioneers,  one 
beaver  pond  covering  100  acres  or  more.  Works  of  an  en- 
gineering sort  have  been  performed  by  them,  such  as  the 


GEOLOGICAL  WORK  OF   ORGANISMS  185 

cutting  of  water  channels  of  considerable  length  for  the 
transport  of  wood  for  their  dams. 

164.  Guano  and  phosphatic  rocks. — Guano  is  composed 
largely  of  the  excrement,  bones,  and  other  remains  of  birds, 
and  to  some  extent  of  other  animals.     Such  deposits  occur 
in  dry  climates,  where  the  materials  are  not  removed  by 
leaching.     They  are  found  in  Peru  and  other  rainless  re- 
gions.    The  places  of  deposit  are  breeding  and  cemetery 
grounds  for  these  creatures,  and  were  brought  to  notice  by 
Humboldt  in  1804  and  by  Liebig  in  1840.     Some  ancient 
phosphatic  lime  rocks  of  great  commercial  value  have  been 
formed  by  the  leaching  of  such  deposits  of  guano,  which 
rested  upon  the  beds  of  calcium  carbonate. 

GEOLOGICAL  WORK  OF  MAN 

165.  Without  man  the  geological  forces  would  seem  to 
be  aimless.    With  man  they  assume  a  high  dramatic  interest. 
But  we  must  not  here  forget  that  man  himself,  the  high- 
est of  land  animals,  is  also  a  geological  force  of  prime 
importance,  able  to  subdue  the  earth,  to  control  and  direct 
in  a  large  measure  its  manifestations  of  energy,  and  at  length 
profoundly  to  modify  its  character.     It  is  not  easy  to  clas- 
sify the  body  of  facts  to  which  we  now  refer.    They  form  a 
network  and  are  so  intimately  related  and  delicately  bal- 
anced that  to  modify  at  one  point  may  introduce  a  long 
series  of  changes.     We  may  for  convenience  take  the  fol- 
lowing heads : 

166.  Planting  and  destruction  of  trees.— The  first  act  of 
man  in  a  new  country,  if  it  be  covered  with  forest,  is  to  cut 
away  trees  and  clear  the  land  for  tillage.     He  may  carry 
this  so  far  as  to  impair  the  spongy,  shaded  reservoirs  of  the 
rain  which    forest  grounds  afford,  and  deluge  the  lower 
lands  with  floods.     The  climate  may  be  modified  in  ways 
not  yet  well  understood,  and  the  conditions  of  vegetable 
and  animal  life  be  revolutionized.      On  the  other  hand, 
man  turns  his  attention  to  the  foresting  of  prairies  and  to 


186  GEOLOGY 

the  growth  of  trees  on  cold  and  barren  slopes  and  on  hill 
tops  which  ought  never  to  have  been  denuded  of  their 
forest  cover. 

167.  Exposure  by  agricultural  processes. — The  plow  and 
harrow  destroy  the  protective  coat  of  plants  and  continu- 
ally pulverize  the  soil  and  enable  rivulets  and  winds  to 
sweep  it  away.     A  blinding  storm  of  fine  earth  may  be 
raised  over  a  plowed  field  while  all  the  surrounding  air  is 
clear.     The  creep  of  soils  by  frost  and  gravitation  is  thus 
made  easy,  and  the  sidehill  plow  and  other  implements 
give  the  soils  a  direct  push  toward  the  valley  bottoms. 

168.  Excavations  and  borings. — The   opening  of    wells 
affects  springs  whose  normal  issue  is  at  other  points,  and 
the  piercing  of  reservoirs  of  gas  may  relieve  stupendous 
pressures  which  would  tend  to  work  changes  in  the  subter- 
ranean rocks.     Innumerable  quarries  and  railway  cuts  are 
opened,  by  which  rocks  are  removed,  and  sections  are 
made   which   vie  with    natural    rock    exposures  in  their 
value  for  geological  study.     Such  are  the  Hoosac  Tunnel, 
4  miles  long,  and  the  wonderful  St.  Gothard  Tunnel,  which 
pierces  the   Alps  for  a  distance  of  9  miles.     Even  more 
extensive  in  certain  areas  are  mining  excavations,  which 
may  extend  to  the  depth  of  a  mile,  and  by  which  much 
of  the  rock  under  many  acres  of  surface  may  be  removed. 
Such  a  case  is  afforded    by  the   city   of    Scranton,   Pa. 
Earth  movements  and  dangerous   subsidences   sometimes 
occur  in  this  manner. 

169.  Modifications  of  the  flow  of  water. — Some   of  the 
reservoirs    and   feeders   of  the   Erie   Canal   on   the   high 
ground  of  the  central  New  York  plateau  have  served  to 
turn  the  drainage  of  many  square  miles  from  the  Susque- 
hanna  to  the  Mohawk  basin.     The  building  of  milldams 
floods  many  tracts  of  low  ground  and  has  been  a  most  fruit- 
ful source  of  legal  actions.    Eaceways  are  cut,  river  channels 
are  deepened  and  straightened  or  even  diverted  from  their 
natural  courses.     Thus  a  stream  in  the  Bernese  district  of 


GEOLOGICAL  WORK  OF  ORGANISMS  187 

Switzerland  was,  in  1714,  turned  by  a  tunnel  into  Lake 
Thun,  whereas  it  had  entered  the  Aar  at  a  considerable 
distance  below  the  lake.  The  roof  of  the  tunnel  soon  fell 
in,  and  the  river  now  passes  through  an  imposing  gorge  at 
that  point  and  has  built  delta  lands  several  acres  in  extent 
into  the  lake.  It  is  of  additional  interest  to  note  that  the 
hand  of  man  has  but  recalled  the  stream  to  its  old  course 
in  preglacial  times.  Everywhere  in  Switzerland,  as  along 
the  Ehone  and  down  the  slopes  of  innumerable  alluvial 
cones,  the  rivers  and  torrents  are  "  rectified  "  and  kept  by 
retaining  walls  from  devastating  adjacent  homes  and  fields. 
The  levees  and  jetties  of  the  Mississippi  Kiver  offer  a  fur- 
ther significant  illustration  of  the  geological  activity  of 
man.  Even  more  striking,  perhaps,  is  the  Chicago  Drain- 
age Canal.  For  a  distance  of  28  miles  a  broad  channel  40 
feet  deep  has  been  cut,  partly  through  drift  and  partly 
through  the  solid  rock.  By  this  means  it  is  possible  to 
divert  a  considerable  quantity  of  Lake  Michigan  waters 
from  the  St.  Lawrence  to  the  Mississippi.  This  is  also 
interesting  as  a  partial  return  at  least  to  glacial  conditions 
of  drainage.  A  great  aggregate  of  artificial  drainage  has 
also  been  effected  for  agricultural  and  for  sanitary  pur- 
poses. Over  hundreds  of  thousands  of  acres  the  flow  of 
waters  toward  the  sea  is  hastened.  Thus  vegetation  is 
much  modified,  and,  as  stated  by  Coulter,  the  plants  which 
man  desires  flourish,  but  many  others  suffer  or  disappear. 
A  further  diversion  of  natural  flow  is  found  in  the  irriga. 
tion  so  extensively  practiced  in  dry  regions.  A  dam  is  now 
under  construction  at  the  cataract  of  the  Nile,  by  which  a 
great  body  of  water  will  be  kept  in  reserve  for  the  irriga- 
tion of  lower  Egypt.  This  may  affect  the  climate  and 
revolutionize  the  agriculture,  and  perhaps  indeed  the  en- 
tire social  and  political  development  of  the  country.  In 
California  a  million  or  more  acres  of  land  are  "  under  the 
ditch  " — that  is,  subject  to  irrigation — and  about  the  same 
area  in  Colorado. 


188  GEOLOGY 

170.  Changes  made  by  man  on  the  seashore. — On  all  civi- 
lized shores  man  co-operates  extensively  with  the  sea  in  its 
activity,  as  in  the  great  variety  of  harbor  constructions, 
preservation  of  dune  surfaces,  and  the  reclaiming  of  the  salt 
marshes.     For  generations  Englishmen  have  set  themselves 
to  reclaim  the  fens  of  Lincolnshire,  and  400,000  acres  of 
fertile  fields  and  many  thriving  towns  testify  to  their  suc- 
cess ;  nearly  1,000,000  acres  have  thus  been  recovered  in  the 
Netherlands.      Even  to  deep-sea  deposits  man  makes  his 
unfailing  contribution,  melancholy  in  interest,  if  insignifi- 
cant in  quantity.     We  have  hinted  at  the  widespread  influ- 
ence of  man  upon  the  surface  materials  and  life   of  the 
globe.     A  fuller  discussion  belongs  to  Physical  Geography. 

SUMMARY  VIEW  OF  GEOLOGICAL  FORCES 

171.  We  have  now  passed  in  review  the  various  ways  in 
which  energy  is  applied  in  changing  the  face  of  the  earth. 
The  atmosphere  spreads  everywhere,  doing  its  destructive 
work.     The  distribution  of  water  is  almost  as  general,  even 
on  the  land  and  beneath  the  surface.     Glaciers  either  are 
or  have  been  the  means  of  change  over  immense  areas. 
No  part  of  land  or  sea  is  without  some  organic  population, 
and  volcanic  and  earth  movements  are  the  product  of  forces 
which  never  rest.     Of  the  uncounted  illustrations  of  all 
these  processes  we  have  here  recorded  a  few,  but  others 
may  be  found  by  the  student  in  whatever  part  of  the  world 
he  chances  to  be ;  and  nothing  will  make  geological  changes 
seem  so  real  as  to  search  them  out  and  see  them  going  on 
under  our  own  eyes.     These  forces  are  sometimes  classified 
as  aqueous,   igneous,   and    organic.      While   such  groups 
partly  correspond  to  the  facts,  we  need  not  put  stress  on 
them,  for  even  the  aqueous  agents  wholly  depend  on  heat 
for  their  efficiency.    It  is  indeed  the  sun's  heat,  rather  than 
the  interior  heat  of  the  earth,  but  it  is  heat,  and  without  it 
evaporation  and  the  subsequent  processes  of  aqueous  ero- 
sion could  not  take  place.     So  also  igneous  work  mingles 


GEOLOGICAL  WORK  OP  ORGANISMS  189 

with  that  of  water,  as  in  the  explosive  eruptions  of  volca- 
noes, in  hot  springs,  and  the  consolidation  of  volcanic  ash. 
Organic  work  also  stands  in  a  way  by  itself,  but  more  truly 
considered  is  dependent  upon  heat  and  water  for  its  effect- 
iveness. Thus  such  definitions  involve  error  and  are  at 
least  incomplete.  We  might  with  some  writers  speak  of 
surface  and  subterranean  forces,  but  here  again  we  come 
into  confusion,  as,  for  example,  when  we  learn  that  the 
moon  or  atmospheric  pressures  have  more  or  less  to  do  with 
earthquakes.  Perhaps  it  is  better,  as  in  the  foregoing  pages, 
to  single  out  a  few  great  kinds  of  agents,  such  as  streams, 
glaciers,  the  ocean,  volcanoes,  etc.,  each  doing  a  variety  of 
things  and  all  combining  with  each  other  in  numberless 
ways,  both  in  working  and  in  results.  Thus  we  go  over  the 
whole  amid  seeming  confusion,  but  at  length  come  out 
with  a  true  appreciation  of  the  orderly  workings  of  a  mul- 
titude of  apparently  diverse  causes  toward  the  harmony  of 
the  world  and  its  fitness  for  intelligent  beings. 

The  incessant  movement  of  materials  on  the  surface  of 
the  globe  is  one  of  the  lessons  which  the  student  has 
learned.  This  is  an  important  preparation  for  the  later 
study  of  geological  history,  in  which  we  must  trace  the 
course  of  changes  during  inconceivable  periods  of  past 
time.  One  great  law  of  such  changes  it  will  be  profitable 
for  us  at  once  to  appreciate — namely,  the  relation  that 
holds  between  uplift  and  degradation.  Elevation  and 
denudation  go  hand  in  hand,  and  balance  each  other  in  a 
remarkable  manner.  If  the  lands  are  raised  to  a  great 
height,  either  by  oscillation  or  by  folding,  all  the  processes 
of  denudation  become  at  once  powerful.  Streams  flow 
more  rapidly,  frosts  are  more  constant  and  effective,  gla- 
ciers form,  and  in  some  cases  volcanoes  may  be  an  accom- 
paniment of  elevation.  As  the  height  of  the  land  is  re- 
duced the  erosive  agents  lose  power,  and  moderate  alti- 
tudes result.  Thus  an  equilibrium  is  maintained  between 
the  forces  of  degradation  and  uplift.  If  continents  were 


190  GEOLOGY 

very  high,  they  would  be  too  cold  and  the  air  too  rarefied 
for  organic  life.  If  they  were  too  low,  there  would  be  no 
variety  of  environment,  little  beauty  of  scenery,  little  dif- 
ference in  organic  groups,  and  a  small  degree  of  individual- 
ity among  the  nations  of  the  earth.  It  has  been  well  said 
that  the  highest  progress  goes  with  a  diversified  physical 
geography  ;  such  geography  is  the  product  of  the  network 
of  geological  forces,  acting  from  the  earliest  ages  of  our 
planet's  history. 


PART  II 
STRUCTURAL  GEOLOGY 


CHAPTEK   XI 

THE    ROCK-FORMING   MINERALS 

172.  Introductory  statement. — We  now  turn  our  attention 
to  the  constitution  of  the  earth's  crust.     We  must  learn 
something  of  the  chemical  and  mineral  composition  of  its 
rocks,  and  something  of  the  forms,  small  and  great,  which 
these  rocks  assume.     In  Part  I  we  studied  the  geological 
energies,  with  some  reference  to  the  resulting  forms.    Here 
we  examine  chiefly  the  forms,  looking  back  incidentally  to 
the  forces  concerned  in  making  them. 

We  shall  pursue  our  way  from  the  smaller  to  the  greater 
elements  of  the  earth's  structure.  All  matter  consists,  so 
far  as  we  know,  of  certain  elementary  substances.  Out  of 
the  elements  minerals  are  formed,  and  these  in  turn  are 
combined  to  make  rocks.  Eocks  are  of  various  kinds,  ac- 
cording to  the  minerals  that  form  them  and  the  forces  that 
affect  them. 

THE  BOCK-FORMING  MINERALS 

While  the  number  of  minerals  is  very  great,  a  small 
number  compose  the  bulk  of  the  rocks  of  the  earth's  crust. 
It  is  with  these  that  we  are  chiefly  concerned  in  the  ele- 
mentary study  of  geology. 

173.  Chemical  or  unresolved  elements. — Of  these,  about 
70  are  thus  far  known  to  chemistry.     More  than  97  per 

191 


192  GEOLOGY 

cent  of  the  earth's  crust  consists  of  9  of  these.  Of  the 
9,  3  are  non-metallic — oxygen,  silicon,  and  carbon.  Oxy- 
gen is  the  most  abundant  of  all,  making  21  per  cent  of 
the  atmosphere,  88.89  per  cent  of  water,  and  50  per  cent  of 
the  rocks — that  is,  of  such  as  are  open  to  observation. 
Silicon  comes  next,  forming  one  fourth  of  the  weight  of  the 
earth's  crust.  The  other  6,  which  are  the  most  important 
metals  from  the  geological  point  of  view,  are  aluminum, 
calcium,  magnesium,  potassium,  sodium,  and  iron.  The 
following  elements  have  in  general  minor  importance,  but 
in  certain  conditions  or  combinations  become  noteworthy 
— sulphur,  hydrogen,  chlorine,  phosphorus,  fluorine,  man- 
ganese, and  barium.  Thus  sulphur  and  hydrogen  are  fre- 
quent products  of  volcanic  action,  and  sulphur,  like  car- 
bon, is  peculiar  in  sometimes  forming  rock  masses  without 
much  admixture  of  other  materials.  Chlorine  has  interest 
as  helping  to  form  common  salt,  which  in  turn  occurs  as 
rock  beds  and  is  abundant  in  sea  water.  Phosphorus  has 
economic  value  in  rocks  of  limited  extent. 

174.  Definition. — A  mineral  is  matter  which  has  a  definite 
chemical  composition,  and  commonly  a  specific  geometrical 
form.     Thus  quartz  and  calcite  are  common  minerals  which 
possess  both  these  properties. 

175.  Properties  of  minerals. — Minerals  are  described  and 
identified  by  virtue  of  certain  qualities  which  they  possess. 
Many  of  these  a  mineral  shares  in  common  with  others,  but 
each  has  its  unique  aggregate  of  characters.     The  chief 
distinctions  of  this  nature  are  the  following  : 

(1)  Composition. — This  has  to  do  with  the   elements 
which   make   up  the  mineral.     We  may  make  a  definite 
analysis  of  it,  or  may  use  terms  which  express  its  chief 
character,  such   as  metallic,  non-metallic,  hydrated,  sili- 
ceous, carbonaceous,  etc. 

(2)  Crystalline  form. — Most  minerals  have  this  prop- 
erty, which  means  that  they  are  bounded  by  plane  surfaces 
of  various  form  and  arrangement.    There  are  six  chief  kinds 


THE  ROCK-FORMING  MINERALS  193 

of  arrangement,  called  systems.     The  science  which  deals 
with  them  is  termed  crystallography. 

(3)  Hardness. — There  are  all  degrees  of  this  property, 
and  ten  well-known  minerals  have  been  agreed  upon  as 
forming  a  standard  scale  with  which  all  others  may  be  com- 
pared.    Of  the  ten,  Xo.  1  is  very  soft  and  No.  10  is  the 
hardest  mineral  known.     The  scale  is  as  follows : 

1.  Talc.  4.  Fluorite.  7.  Quartz.  9.  Corundum. 

2.  Gypsum.         5.  Apatite.  8.  Topaz.  10.  Diamond. 

3.  Calcite.  6.  Orthoclase  (feldspar). 

Of  these  minerals,  a  piece  of  gypsum  will  scratch  talc, 
but  in  turn  is  scratched  by  calcite.  Any  other  mineral  of 
which  the  same  is  true  is  said  to  have  a  hardness  of  2.  If 
hardness  falls  between  two  numbers — as,  for  example,  a 
mineral  which  scratches  orthoclase  but  is  scratched  by 
quartz — its  hardness  is  said  to  be  between  6  and  7. 

(4)  Luster. — Certain  minerals  are  said  to  be  metallic  in 
appearance,  vitreous  or  glassy,  pearly  or  silky,  as  the  case 
may  be. 

(5)  Specific  gravity. 

(6)  Cleavage. — The  property  of  splitting  along  one  or 
more  planes.     Calcite  cleaves  in  three  directions,  feldspar 
in  two,  mica  in  one,  while  quartz  does  not  possess  this 
property.      Cleavage  is  often  more  perfect  in  one  plane 
than  in  others,  and  the  cleavage  planes,  when  two  or  more, 
like  the  crystalline  faces,  intersect  each  other  at  a  definite 
angle  for  the  given  mineral. 

(7)  Streak. — This   refers  to   the  color   of  the  mineral 
when  reduced  to  powder,  and  is   commonly  learned  by 
scratching  the  surface  with  a  hard  point,  as  of  a  steel  blade, 
or  by  rubbing  the  mineral  on  rough  porcelain. 

(8)  Properties  depending  on  heat ;  as  fusibility. 

(9)  Properties  depending  on  the  senses ;  as  taste,  smell, 
feeling. 

176.  Mineralogy. — The  discussion  of  the  properties  and 
classification  of  minerals  belongs  to  the  science  of  mineral- 


194:  GEOLOGY 

ogy,  which  is  inclusive  of  crystallography.     We  here  have 
place  but  for  a  short  account  of — 

177.  The  principal  rock-forming  minerals.— These  are  sil- 
ica and  the  silicates,  the  carbonates  and  other  carbonaceous 
minerals,  gypsum,  chloride  of  sodium  (common  salt),  and 
the  iron  compounds.     These  are  the  only  minerals  which 
form  great  masses  of  rock,  though  others  may  have  eco- 
nomic value  or  scientific  interest. 

178.  Silica. — Silica  is  the  only  known  oxide  of  silicon, 
and  in  its  most  common  form  is  called  quartz.     It  takes 
the  form  of  hexagonal  crystals  or  is  massive  and  ranks  7 
in  the  scale  of  hardness,  being  the  hardest  mineral  with 
which  the  student  will  commonly  meet  in  the  field.    It  can 
usually  be  recognized  by  one's  inability  to  scratch  it  with 
the  point  of  a  knife  blade.     It  has  no  cleavage,  and  when 
pure  is  transparent,  being  then  known  as  rock  crystal.     It 
is  insoluble  in  water  and  in  most  acids,  but  subterranean 
conditions  are  such  as  to  have  caused  its  solution  exten- 
sively.   Thus  many  fossils  and  mineral  veins  consist  largely 
of  quartz.     Much  quartz  is  milky  in  appearance,  and  the 
presence  of  various  elements  forms  amethyst,  rose  quartz, 
cairngorm,  and  other  varieties.    Ferruginous  quartz  is  dark 
in  color,  owing  to  the   presence  of  iron.     Dark,  massive 
quartz  is  often  known  as  flint  or  chert.     Quartz  is  a  most 
important  constituent  of  rocks,  as  of  granites,  sandstones, 
and  many  others. 

179.  Silicates. — These  minerals  are  of  great  variety  and 
importance,  particularly  in  igneous  rocks.    They  are  formed 
by  the  union  of  silica  with  a  metal  or  base.     Among  the 
principal  rock-forming  silicates  we  have— 

(1)  The  feldspars. — These  are  composed  of  silica  and 
alumina  with  potassium,  calcium,  or  sodium.  Some  feld- 
spars cleave  along  planes  vertical  to  each  other,  and  hence 
are  called  orthoclase.  Their  alkali  is  potash.  Many  feld- 
spars show  oblique  cleavage,  and  are  termed  plagioclase. 
Their  alkalies  are  soda  and  lime.  The  hardness  is  6,  and 


THE  ROCK-FORMING  MINERALS  195 

the  colors  are  variable,  though  not  pronounced.  White, 
gray,  green,  yellow,  and  light  red  are  among  those  that 
occur.  Sometimes  conspicuous  crystals  appear  in  a  ground 
mass  of  other  minerals.  Feldspars  decompose  somewhat 
readily  under  the  various  influences  of  the  atmosphere,  and 
many  clays,  more  or  less  pure,  remain  after  the  more  soluble 
alkalies  have  been  removed. 

(2)  The  micas. — These  are  chiefly  composed  of  silica, 
alumina,  potash,  iron  oxide,  and  water.     Their  most  con- 
spicuous property  is  their  cleavage  into  very  thin  leaves, 
which  are  nearly  transparent.     Micas  have  a  brilliant  luster 
and  a  hardness  between  2  and  3.     They  occur  as  small 
flakes  in   many  rocks,  and   sometimes   in   large   crystals, 
affording  broad  sheets.     Muscovite  is  the  variety  which  is 
known  in  the  arts.     Granite,  gneiss,  and  mica  schist  con- 
tain much  mica,  and  sands  formed  by  the  breaking  down 
of  such  rocks  often  shine  with  its  flakes.     It  may  be  white, 
black,  yellow,  green,  or  brown. 

(3)  Hornblende. — This  silicate  contains  alumina,  magne- 
sia, lime,  and  iron  oxide.     Its  hardness  is  between  5  and  6, 
and  its  color  may  be  black,  dark  green,  or  white.    It  occurs 
in  prismatic  crystals  and  in  slender  radiating  crystals  as 
actinolite,   and  is   found  fibrous   in   asbestos,  which   has 
become  useful  in  the  arts  by  virtue  of  this  quality. 

(4)  Talc,  serpentine,    chlorite. — These   are   silicates   of 
magnesia  which  also  contain  water.     In  taking  up  water 
they  have  been  altered  in  composition,  losing  some  sub- 
stances and  taking  up   others.     They  are  soft  minerals, 
readily  cut  with  a  knife,  and  are  commonly  greenish  in 
color,  with  variations   to  white,  red,  or  yellow,  giving  a 
mottled  effect,  and  thus  making  some  of  them  useful  for 
ornamental  work,  as  serpentine.     Verd-antique  marble  is 
a  mixture  of  serpentine  and  limestone.     Talc  varies  from 
apple  green  to  white,  is  greasy  to  the  touch,  and  occurs 
as  foliated  and  massive.     Massive  talc  is  steatite  or  soap- 
stone. 


196  GEOLOGY 

180.  Carbonates. — These  are  formed  by  the  union  of  car- 
bon dioxide  with  a  base.     The  most  important  of  these  is 
calcite  or  carbonate  of  lime,  which  is  the  principal  sub- 
stance of  all  limestones,  and  enters  largely  into  the  struc- 
ture of  marine  creatures.     It  thus  forms  a  great  number  of 
the  fossils  .which  are  preserved  in  rock  strata.     It  is  an 
essential  element  of  soils,  and  vast  amounts  of  it  are  in 
solution  in  the  sea.     It  is  3  in  the  scale  of  hardness,  and 
may  thus  always  be  easily  distinguished  from  quartz,  which 
in  appearance  sometimes  resembles  it.     It   is  more  com- 
monly colorless  or  white,  but  may  be  red,  gray,  yellow,  or 
black.     It  effervesces  vigorously  with   cold    dilute  hydro- 
chloric acid,  and  may  thus  be  detected  when  scattered  in 
small  quantities  in  rocks  other  than  limestone. 

Dolomite  is  a  carbonate  of  calcium  with  magnesium. 
Dana  gives  the  proportions  thus :  calcium  carbonate  54.4, 
magnesium  carbonate  45.6  =  100.  Dolomite  looks  like 
calcite,  but  does  not  effervesce  with  cold  dilute  acid. 
Massive  dolomite  forms  rocks  of  considerable  extent,  and 
is  often  called  magnesian  limestone.  Some  dolomites  form 
hydraulic  cement. 

181.  Other  carbonaceous  minerals. — Chief  among  these 
are  the  varieties  of  coal  and  mineral  oil. 

Coal. — This  substance  is  the  product  of  vegetable  accu- 
mulations in  different  periods  of  the  earth's  history.  The 
least  changed  member  of  the  series  is  peat,  of  which  an 
account  has  already  been  given.  The  next  is  lignite,  some- 
times incorrectly  called  "  brown  coal,"  which  retains  much 
of  its  woody  structure.  Bituminous  coal  exhibits  a  cubical 
fracture,  burns  with  much  flame  and  smoke,  and  contains 
65  to  85  per  cent  of  carbon.  Anthracite  coal  is  hard  and 
lustrous,  burns  with  little  flame,  and  commonly  contains 
90  to  95  per  cent  of  carbon.  Graphite  (black  lead)  is  prop- 
erly included  here,  as  having  passed  a  stage  beyond  anthra- 
cite in  the  loss  of  its  volatile  materials.  Like  the  diamond, 
graphite  is  pure  carbon,  though  the  two  differ  so  much  in 


THE  ROCK-FORMING  MINERALS  197 

appearance.  More  generally  graphite  and  anthracite  are 
found  among  the  older  rocks,  and  bituminous  and  lignitic 
coals  always  occur  in  strata  of  moderate  geological  an- 
tiquity. 

Cannel  coal  is  dense,  lusterless,  breaks  unevenly,  and 
burns  with  a  bright  flame.  Little  or  no  trace  of  plant 
structure  remains  in  it. 

The  coals  might  with  some  propriety  be  described  as 
rocks  rather  than  as  minerals,  since  their  chemical  compo- 
sition is  variable,  and  they  thus  fall  short  of  the  definition 
given. 

182.  Sulphates. — The  only  important  rock-forming  com- 
pound of  sulphur  is  gypsum.    It  contains  about  20  per  cent 
of  water.    When  pure,  it  is  often  crystallized,  and  is  known 
as  selenite.     It  is  frequently  transparent,  pearly  in  luster, 
and  cleaves  into  thin  leaves.     Sometimes  it  is  fibrous,  and 
is  called  satin  spar.     It  is  often  massive,  and  is  ground  for 
use  as  a  fertilizer.     White  and  pure  massive  gypsum  is  ala- 
baster.    If  the  water  be  driven  off  by  heating  and  the  resi- 
due reduced  to  powder,  it  is  called  plaster  of  Paris.     Gyp- 
sum is  often  associated  with  beds  of  rock  salt. 

183.  Chloride  of  sodium  (common  salt). — This  mineral  has 
great  economical  value,  and  was  formerly  derived  mainly 
from  sea  water.     In  modern  times,  however,  large  beds  of 
it  have  been  found  associated  with  the  stratified  rocks  of 
various  countries. 

184.  Iron  compounds. — Those  which  are  important  geo- 
logically are  four  in  number : 

(1)  Magnetite. — It  is  black,  either  crystalline  or  massive, 
magnetic,  and  is  an  important  iron  ore.     It  is  composed  of 
iron,  72.4  per  cent ;  oxygen,  27.6.     The  streak  is  black. 
Particles  may  frequently  be  withdrawn  by  a  magnet  from 
seashore  or  glacial  sand. 

(2)  Hematite. — Occurs  as   crystalline,  or  massive  and 
earthy ;  forms  a  red  powder.     Red  ochre  is  the  earthy  vari- 
ety.    It  contains  iron,  70  per  cent ;  oxygen,  30  per  cent. 

14 


198  GEOLOGY 

(3)  Limonite. — This  is  a  brownish  or  yellowish  ore,  con- 
taining iron,  oxygen,  and  about  15  per  cent  of  water.     The 
yellow  sort  is  yellow  ochre.     An  impure  earthy  variety  of 
limonite  is  found  in  swamps,  and  hence  called  bog  iron  ore. 

(4)  Pyrites. — Magnetite,   hematite,   and    limonite    are 
oxides  of  iron,  but  pyrites  is  a  sulphide.     It  contains  iron, 
46.7  per  cent ;  sulphur,  53.3  per  cent.     Occurs  massive  and 
in  cubical  crystals,  is  pale  or  bronze  yellow  in  color,  and 
widely  distributed.     It  has  often   been   taken  for  silver, 
gold,  or  copper.     It  has  little  value,  is  not  used  for  making 
iron,  but  is  sometimes  employed  for  the  manufacture  of 
oil  of  vitriol  or  sulphuric  acid.     Occasionally  it  contains 
enough  gold  to  pay  for  working. 


CHAPTER  XII 

COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS 

185.  HAVING  passed  in  review  some  of  the  more  impor- 
tant minerals,  we  now  turn  to  the  rocks  which  they  form. 
In  a  few  cases  a  single  mineral  occurs  in  large  masses,  and 
may  be  called  a  rock  ;  but  most  rocks  are  a  mixture  of  two 
or  more  minerals.     Often  two  or  three  characteristic  min- 
erals make  up  the  bulk  of  a  rock,  while  a  large  number  of 
others  are  present  in  small  quantities.     The  minerals  in 
rocks  may  be  perfect  crystals,  but  more  often  occur  as  crys- 
talline fragments,  or  in  such  minute  particles  as  to  require 
chemical  tests  or  microscopic  observations  for  their  detec- 
tion.    The  science  which  thus  deals  with  the  making  up  of 
rocks  is  petrography.     It  is  a  department  of  geology,  and 
this  chapter  will  present  a  few  of  the  more  simple  and  com- 
mon facts.     Rocks  may  be  classified  as  fragmental,  igneous, 
and  metamorphic.     Before  taking  these  up  it  will  be  well 
to  define  a  number  of  terms  used  to  describe  the  appearance 
and  character  of  rocks. 

186.  Descriptive  terms. — A  rock  is  crystalline  if  com- 
posed of  whole  or  partial  crystals,  whose  lustrous  faces  often 
shine  on  the  wall  of  a  fracture.     The  adjectives  compact, 
amorphous,  and  massive  are  sometimes  loosely  used.     Prop- 
erly, amorphous  refers  to  the  absence  of  the  crystalline  con- 
dition, massive  to  the  absence  of  large  planes  of  division, 
especially  planes  of  stratification,  and  the  particles  of  a 
compact  rock  are  not  seen  by  the  unaided  eye.     A  friable 
rock  crumbles  readily,  as  under  the  pressure  of  the  fingers. 

199 


200 


GEOLOGY 


Granular  rocks  are  composed  of  nearly  equal  grains  of  one 
or  more  minerals,  and  vitreous  refers  to  a  glassy  appearance. 
A  shall/  rock  splits  readily  into  thin,  smooth  layers  along 
planes  of  stratification.  Foliated  or  schistose  rocks  also  split 
readily,  usually  into  wavy  layers,  because  of  leaves  or  scales 
of  a  cleavab'le  mineral  like  mica.*  Cellular  describes  a  rock 
with  small  rounded  cavities,  such  as  bubbles  of  air  or  gas 
cause  in  lava.  If  complete  crystals  of  one  mineral  appear 
imbedded  in  a  ground  mass  or  matrix  of  others,  the  rock  is 
porpliyritic.  If  a  rock  is  made  up  of  small  rounded  grains 
like  the  roe  of  fish,  it  is  oolitic.  If  the  grains  have  the  size 
of  peas,  the  rock  is  called  pisolitic. 


FIG.  90.— Cellular  structure. 


Various  terms  scarcely  needing  definition  refer  to  a 
more  abundant  or  characteristic  element  in  the  composi- 
tion of  rocks.  Thus  calcareous  rocks  have  a  considerable 
proportion  of  lime ;  argillaceous  rocks  contain  much  sili- 
cate of  alumina  or  clay ;  ferruginous  rocks  are  so  called 

*  See,  on  the  schistose  rocks,  p.  213.  For  the  distinction  between 
schistose  structure  and  slaty  cleavage,  see  p.  241. 


COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS    201 

from  the  presence  of  iron.  Similarly  we  use  the  words 
siliceous,  quartzose,  saliferous  (salt-bearing),  micaceous, 
and  carbonaceous.  Likewise  the  whole  series  of  color 


terms  is  needed  in  the  study  of  rocks.  White,  black,  and 
many  shades  of  yellow,  brown,  red,  blue,  and  green  are 
found.  A  true  eye  for  color  is  desirable  for  one  who  would 
go  far  in  the  study  of  rocks  or  be  able  to  describe  them. 

THE  FRAGMESTTAL  EOCKS 

187.  These  rocks  are  so  called  because  they  consist  of 
fragments,  usually  small  but  sometimes  large,  of  older 
rocks.  They  are  commonly  deposited  in  water,  and  hence 
are  often  called  sedimentary  or  aqueous.  They  may  be 
laid  down  in  the  bed  of  the  sea  or  of  a  lake,  or  along  the 
course  of  streams.  Masses  of  talus  or  of  volcanic  ash 
unmodified  by  water  are  examples  of  non-sedimentary 
fragmental  rocks.  This  class  of  rocks  may  have  any  chem- 
ical constitution,  depending  upon  the  character  of  the 
masses  from  which  they  were  derived.  We  take  first — 


GEOLOGY 


1.  The  Sand  and  Gravel  Group 

188.  Sand.— This  consists  of  broken  particles  of  rock  of 
any  kind.  The  fragments  are  of  appreciable  size,  but  not 
larger  than  is  expressed  by  the  term  grain.  More  com- 
monly most  of  the 
grains  are  of  quartz, 
because  this  mineral 
is  hard  and  survives 
the  wear  of  waves 
and  currents.  Even 
if  sands  show  various 
colors,  a  low-power  mi- 
croscope will  reveal  a 
surprising  predomi- 
nance of  the  quartz. 
The  grains  will  also 
be  angular  or  round- 
ed and  battered,  ac- 
cording to  the  dis- 
tance traveled  or  the 
violence  of  the  waves 
to  which  they  have  been  subjected.  We  distinguish  river 
sand,  beach  sand,  whether  of  lakes  or  ocean,  eolian  sand, 
which  mainly  is  derived  from  beaches,  glacial  sand,  and 
volcanic  sand. 

Sandstone.— A  sandstone  is  a  mass  of  sand  whose  grains 
are  more  or  less  firmly  bound  together  by  some  kind  of 
cement.  Frequently  the  cement  is  an  oxide  of  iron,  giv- 
ing a  red  color  to  the  rock.  Sometimes  the  cement  is  a 
deposit  of  quartz  among  the  original  quartz  grains,  form- 
ing a  most  durable  rock.  Other  sands  are  cemented  by 
carbonate  of  lime.  Even  recent  glacial  sands  are  some- 
times changed  into  firm  rock  by  infiltration  of  lime  waters. 
This  cement  being  soluble,  calcareous  sandstones  are  less 
durable. 


FIG.  92.— Conglomerate. 


COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS    203 

Gravel. — This  name  is  given  to  a  mass  of  pebbles  or  of 
coarse  sand  mingled  with  pebbles.  Beds  of  gravel  often 
alternate  with  beds  of  sand  and  beds  of  clay,  pointing  back 
to  alternating  seasons,  or  wet  and  dry  periods,  with  swift 
and  sluggish  flow  of  currents ;  or,  in  the  case  of  the  sea, 
the  gravel  may  lie  on  the  upper  margin  of  the  beach  and 
the  sand  below. 

Conglomerate. — Thus  we  designate  the  rock  formed  by 
the  consolidation  of  gravel  by  cementing  and  pressure.  If 
the  pebbles  are  rounded  by  water  action,  the  resulting  con- 
glomerate is  sometimes  called  a  pudding  stone.  Such  is 
the  Oneida  conglomerate  of  central  New  York,  the  Lower 
Carboniferous  conglomerate  of  "  Rock  City "  near  Olean, 
N.  Y.,  or  the  very  ancient  Eoxbury  conglomerate  in  Boston. 


ear  Highgate  Palls,  Vt. 


Breccia. — A  breccia  is  a  rock  formed  by  consolidating  a 
mass  of  angular  rock  fragments.  Thus  we  may  have  talus, 
volcanic,  and  fault  breccias,  according  to  the  mode  of  ori- 
gin of  the  fragments.  In  the  last  case  the  fragments  are 
formed  by  the  crushing  that  sometimes  takes  place  during 


204  GEOLOGY 

movements  of  dislocation  along  a  fracture  plane.  Such 
rocks  sometimes  cut  and  polish  well,  and  the  angular  pieces 
show  finely  in  ornamental  work. 

2.  The  Clay  Group 

189.  True  clay  is  a  silicate  of  aluminum  formed  in 
great  abundance  by  the  decomposition  of  f  eld  spathic  rocks. 
It  is  fine,  smooth,  plastic,  and  of  various  colors,  depending 
upon  slight  admixtures  of  other  substances.  The  term 
clay  is,  however,  loosely  applied  to  a  great  variety  of  muds, 
consisting  largely  of  finely  pulverized  rocks  of  many  kinds, 
in  which  some  true  clay  is  usually  present.  Such  are  many 
clays  of  the  sea  bottom,  formed  of  the  finer  land  Avaste,  the 
clays  of  ancient  lake  basins,  now  drained,  and  many  clays 
of  glacial  origin.  The  last  are  often  blue  below,  but  yellow 
or  reddish  in  their  oxidized  upper  portions. 

Kaolin  is  a  pure,  often  white,  oily-feeling  clay,  valuable 
for  pottery.  Brick  clay  is  an  impure  variety,  containing 
iron,  to  which,  when  oxidized  in  burning,  the  color  of  brick 
is  often  due.  Fire  clays  are  used  for  bricks  when  walls  or 
inclosures  are  desired  which  will  endure  great  heat.  They 
are  nearly  free  from  lime,  alkalies,  and  iron.  A  clay  min- 
gled with  calcium  carbonate  forms  marl.  A  loam  consists 
of  clay  with  sand  and  some  vegetable  matter,  as  in  many  of 
the  best  and  most  easily  worked  soils. 

Shale. — A  shale  is  a  clay  or  mud  rock  which  splits  into 
thin  leaves  along  the  planes  of  deposition.  These  rocks 
show  great  diversity  of  composition,  according  as  lime,  iron, 
or  other  substances  are  present.  They  may  contain  fine 
sand,  and  thus  grade  into  sandstones.  Indeed,  we  may  find 
a  perfect  series  from  the  finest  shales  to  the  coarsest  con- 
glomerates. Some  shales  contain  much  carbonaceous  mat- 
ter, and  are  used,  as  in  Scotland,  New  South  Wales,  and  else- 
where, for  the  production  of  gas  and  oil.  If  the  calcareous 
matter  is  abundant  in  shales,  they  grade  into  limestones, 
forming  as  perfect  a  transition  as  in  the  case  of  sandstones. 


COMPOSITION  AND  MINUTE  STRUCTURE   OF  ROCKS    205 

3.  The  Limestone  Group 

190.  These  rocks  are  mainly  of  organic  origin,  and  often 
preserve  a  full  record  of  the  marine  life  of  their  period  of 
deposit.  The  principal  mineral  in  them  is  carbonate  of 
lime,  but  many  other  substances  enter  into  them,  so  that 
they  are  various  in  texture,  hardness,  and  color.  More 
commonly  they  are  of  dull  blue,  drab,  or  grayish  hues, 
but  sometimes  they  are  black,  not  infrequently  yellow, 
and  occasionally  white.  They  may  hold  much  clayey 
material  and  thus  graduate  into  shales.  Carbonate  of  lime 
may  mingle  with  sand,  producing  a  rock  of  intermediate 
type. 

Some  limestones  are  crystalline,  while  others  look  like  a 
hardened  mud.  The  crystalline  structure  is  usually  due  to 
changes  during  long  periods  following  the  time  of  forma- 
tion. It  may,  however,  develop  more  rapidly  under  the 
influence  of  pressure  or  heat.  Limestones  are  sometimes 
shaly  and  of  loose  texture,  and  others  are  massive.  When 
limestone  is  burned,  carbon  dioxide  is  driven  off,  leaving 
quicklime.  Some  limestones,  when  freshly  broken,  give  off 
strong  odors,  due  to  the  presence  and  decomposition  of 
organic  matter. 

Certain  varieties  of  limestone  should  receive  particular 
mention.  One  of  these  is  chalk,  a  white,  fine-grained,  soft, 
and  friable  rock.  The  Cretaceous  formation  of  England 
contains  extensive  and  famous  beds  of  chalk.  Similar  de- 
posits are  found  in  Texas,  and  resemble  some  of  the  beds 
of  ooze  in  modern  seas.  A  chalky  accumulation  known  as 
shell  marl,  and  occurring  in  fresh-water  ponds,  has  already 
been  described.  Hydraulic  limestone  is  so  called  because 
when  ground  it  will  "  set "  under  water.  It  contains  vari- 
ous impurities,  such  as  silica,  alumina,  and  sometimes  iron. 
Travertine  is  a  lime  rock  deposited  from  springs.  It  is 
sometimes  called  calcareous  tufa,  and  is  common  on  the 
banks  of  streams  or  springs  that  issue  from  limestone 


206  GEOLOGY 

rocks.     It  frequently  incrusts  or  forms  molds  of  leaves, 
twigs,  and  other  objects. 

THE  IGNEOUS  EOCKS 

191.  General  characters. — The  igneous  rocks  are  always 
unstratified,  though  they  may  lie  between  sedimentary  beds, 
and  thus  have  the  appearance  of  stratification.     They  are 
never  fossiliferous,  though  a  lava  stream  or  bed  of  ash 
might  by  accident  include  organic  forms.     They  are  some- 
times called  massive,  but,  as  we  have  seen,  this  term  is  also 
used  for  water-laid  rocks,  whose  bedding  planes  are  infre- 
quent and  inconspicuous.     Igneous  rocks  are  also  called 
crystalline,  but  some  volcanic  beds  are  not  so,  while  some 
sedimentary  rocks  have  this  character.     But  all  show  signs 
of  the  action  of  heat,  and  the  term  igneous  is  therefore  ap- 
propriate. 

Ancient  as  some  fragmental  rocks  are,  many  of  the 
igneous  masses  are  older,  and  constitute  the  floor  on  which 
the  sediments  rest.  This  basement  formation  would  be 
found  everywhere  if  our  observations  could  go  far  enough 
into  the  crust  of  the  earth.  Igneous  rocks  also  belong  to 
all  geological  periods,  and  their  age  is  determined  by  their 
relation  to  the  fossil-bearing  rocks  with  which  they  are  asso- 
ciated. 

192.  Classification.— In  a  general  way  igneous  rocks  are 
classified  according  to  the  place  of  their  formation.    If  they 
were  formed  in  a  deeply  subterranean  zone,  they  may  be 
called  Plutonic.     If  near  or  upon  the  surface  they  are  vol- 
canic or  eruptive.     But  there  are  all  gradations  between 
the  two.     The  Plutonic  rocks  are  crystalline,  for  the  reason 
that  cooling  proceeded  slowly,  and  the  elements  had  time 
to  arrange  themselves  in  crystalline  forms  before  the  mass 
became  rigid.     Such  rocks  now  often  lie  at  the  surface. 
This  means  that  prolonged  erosion  has  stripped  off  the 
cover  of  overlying  rocks  under  which  they  were  formed. 
The  granites,  or  at  least  many  of  them,  belong  to  this  class, 


COMPOSITION  AND   MINUTE  STRUCTURE   OF  ROCKS    207 

but  are  now  extensively  brought  to  light  in  all  parts  of  the 
world.  The  volcanic  rocks  are  crystalline  in  a  much  less 
degree,  but  often  reveal  the  presence  of  minute  rudimen- 
tary crystals  under  the  microscope.  Obsidian  and  basalt 
are  examples. 

Igneous  rocks  are  also  subject  to  chemical  classification. 
The  broad  principle  of  arrangement  is  the  presence  of  vary- 
ing proportions  of  silica  and  metallic  bases.  If  the  propor- 
tion of  silica  is  as  high  as  60  to  80  per  cent,  the  rock  is 
called  acidic.  Granite  is  an  example  of  a  Plutonic,  acid 
rock.  Obsidian  is  a  volcanic  acid  rock.  If  the  silica  falls 
below  60  per  cent  and  the  metals  are  strongly  present,  the 
rock  is  called  basic.  Basalt  is  a  common  example  of  a 
basic  rock.  A  complete  series,  however,  unites  the  two 
sorts. 

193.  Plutonic  rocks. — Among  the  most  common  and  im- 
portant is  granite.  It  should  be  stated  that  some  granites 
are  thought  to  have  been  formed,  not  by  the  cooling  of 
molten  matter,  but  by  extreme  modification,  or  metamor- 
phism  (section  196),  of  sediments.  The  primary  minerals  in 
ordinary  granite  are  three — quartz,  feldspar,  and  mica.  The 
quartz  may  be  recognized  by  its  glassy  grains,  which  usually 
do  not  show  a  crystalline  structure.  The  feldspar  shows  its 
crystalline  faces,  and  is  generally  pink  or  gray  or  whitish  in 
color.  The  mica  may  be  detected  in  flakes  of  greater  or  less 
size,  according  as  the  rock  is  of  fine  texture  or  coarse.  Other 
minerals  are  present  in  small  degree.  If  hornblende  occurs 
in  place  of  mica,  the  rock  is  a  hornblende  granite.  The 
color  of  granite  varies  much,  particularly  with  the  colors  of 
the  feldspar.  Thus  we  may  compare  the  familiar  red  Scotch 
granite,  with  the  gray  granites  of  Quincy  or  Cape  Ann, 
Massachusetts.  Granites  are  widely  distributed,  and  often 
occur  over  large  areas.  Granite  forms  the  core  or  axis  of 
some  mountain  ranges,  as  in  the  Pyrenees,  Himalayas,  and 
Sierra  Nevada.  It  also  occurs  as  veins  and  in  other  forms 
intruded  into  rocks  of  different  character. 


208  GEOLOGY 

194.  Syenite  is  a  rock  whose  chief  constituents  are  feld- 
spar and  hornblende  without  quartz.     It  also  contains  a 
number  of  accessory  minerals,  and,  like  granite,  has  been 
formed  in  many  periods  of  geological  history.     Hornblende 
granite  was  formerly  called  syenite.     Diorite  is  chiefly  com- 
posed of  hornblende  and  plagioclase  feldspar,  differing  from 
syenite  in  the  character  of  its  feldspar,  which  in  the  latter 
is  orthoclase.     Diorite  is  a  fine-grained  rock,  and  is  not  in- 
frequently called  greenstone.     If  the  rock  contains  some 
quartz  it  is  called  quartz  diorite.     Gabbro  and  diabase  are 
dark-colored  rocks,  of  which  plagioclase  feldspars  are  al- 
ways a  constituent,  resembling  the  basalts  in  composition, 
but  more  fully  crystalline,  being  mainly  of  deep-seated  or 
Plutonic  origin.    They  are  often  called  trap  rock,  and  form 
(diabase)  the  Palisades  of  the  Hudson,  and  occur  extensively 
(gabbro)  in  the  Adirondack  Mountains.    It  is  to  be  remem- 
bered that  some  of  the  Plutonic  rocks  are  acid,  like  granite, 
while  others,  like  the  diabase  and  gabbro,  are  basic.     Many 
other  kinds  of  Plutonic  rocks  occur,  with  endless  varieties 
of  those  here  briefly  described,  but  the  knowledge  of  these 
belongs  to  Petrography. 

195.  The  volcanic  or  eruptive  rocks. — These  also  are  both 
acid  and  basic.     As  before,  we  begin  with  an  illustration 
of  the  more  acid  type,  the  obsidian  or  volcanic  glass.     It 
contains  70  per  cent  or  more  of  silica,  varies  from  green  or 
blue  to  red,  brown,  or  black  in  color,  is  more  or  less  trans- 
lucent, and  breaks  like  bottle  glass.     It  is  an  acid  lava,  so 
rapidly  cooled  that  it  contains  only  minute  or  rudimentary 
crystals.     Sometimes  it  shows  banding,  preserving  the  flow 
structure  of  the  lava.     It  may  be  cellular,  and  if  largely  so, 
becomes  a  volcanic  pumice.     It  occurs,  among  other  places, 
in  Teneriffe,  in  Iceland,  and  forms  Obsidian  Cliff  of  the 
Yellowstone  Xational  Park. 

Ehyolite  and  trachyte  are  "  stony  "  lavas  much  like  ob- 
sidian in  chemical  constitution,  but  having  cooled  more 
slowly,  thus  assuming  a  crystalline  structure,  though  more 


COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS    209 

or  less  of  glass  remains  in  which  the  crystals  are  imbedded, 
especially  in  the  rhyolites.  They  occur  in  great  abundance 
in  the  lava  flows  of  later  geological  times  in  Europe  and 
the  western  United  States.  The  basalts  include  the  more 
basic  eruptive  rocks.  Typical  basalt  is  a  dark  lava,  whose 
crystals  are  microscopic,  or  are  scattered,  forming  a  por- 
phyrite  structure.  Some  basalts  are  glassy,  and  thus  re- 
semble obsidian  in  structure.  If  a  basic  lava  has  cooled 
slowly,  so  as  to  be  coarsely  crystalline,  it  is  called  a  dolerite. 
The  basalts  usually  contain  40  to  50  per  cent  of  silica. 
They  are  widely  distributed  and  form  the  lavas  of  many 
Tertiary  and  modern  eruptions.  They  often  assume  a  co- 
lumnar structure  (section  236). 

THE  METAMOBPHIC  ROCKS 

196.  Metamorphism  is  literally  a  change  of  form,  and 
comprehends  certain  changes  in  rocks,  especially  in  the 
direction  of  hardness  and  crystalline  character.     It  is  not 
used  of  the  ordinary  degrees   of   pressure   and  cementa- 
tion by  which  loose  sediments  are  made  to  cohere,  but  of 
changes  beyond  these,  often  involving  decided  modification 
in  the  character  and  arrangement  of  the  constituent  min- 
erals.    Metamorphic   rocks   may  be   derived  either  from 
sedimentary  or  from  igneous  formations.      Ordinary  con- 
solidated sediments  may  become  quite  hard  and  crystalline, 
and  we  thus  pass  insensibly  to  the  true  metamorphic  types. 
Metamorphic  aqueous  rocks  are  commonly  without  fossils, 
but  in  some  cases  the  changes  have  stopped  short  of  their 
obliteration.     Volatile  matters  are  driven  off,  as  when  soft 
coal  becomes  anthracite. 

197.  Where  metamorphic  rocks  occur.— They  are  found 
more  often  among  the  older  rocks,  but  sometimes  in  those 
of  recent  origin.     The  ancient  rocks  have  been  subject  to 
a  very  long  series  of  modifying  influences  as  compared  with 
the  modern.     In  regions  of  vulcanism,  hot  lavas  have  meta- 


210  GEOLOGY 

morphosed  the  rocks  with  which  they  came  in  contact. 
This  takes  place  on  the  walls  of  dikes,  and  above  and  be- 
low intrusive  sheets,  or  beneath  a  lava  stream.  The  rock 
is  baked,  or  modified  by  hot  vapors  or  heated  waters.  Such 
effects  can  extend  but  a  little  way  from  the  heated  mass, 
and  hence  the  result  is  called  local  metamorphism.  But 
similar  changes  take  place  over  many  thousand  square  miles, 
and  are  due  to  more  general  causes,  as  we  shall  see.  In 
this  case  we  speak  of  regional  metamorphism.  Western 
New  England  and  eastern  Xew  York,  including  Manhat- 
tan Island,  form  a  great  belt  of  metamorphic  rocks.  So 
we  find  them  in  parts  of  the  Adirondack^,  in  northern 
Michigan,  over  great  areas  in  Canada  and  in  the  Highlands 
of  Scotland,  in  the  English  lake  district,  and  in  Wales. 
Local  metamorphism  is  found  in  regions  of  recent  or 
ancient  volcanic  action ;  regional  metamorphism  is  found  in 
areas  of  widespread  disturbance  and  crushing. 

198.  Sedimentary  origin  of  some  metamorphic  rocks. — 
That  some  metamorphic  rocks  were  originally  fragmental 
is  shown  in  a  variety  of  ways.  Occasionally,  though 
rarely,  fossils  are  found  in  them.  This  is  the  case 
near  Eutland,  Vt.,  where  fossils  have  been  discovered  in 
rocks  lying  between  ranges  of  marble  quarries,  and  in 
Bernardston,  Mass.,  where  a  variety  of  metamorphic  min- 
erals is  found  closely  associated  with  Devonian  fossils. 
In  Norway,  also,  a  Silurian  limestone  contains  at  once 
fossils  and  crystals  of  garnet.  The  fossils  show  that  the 
rock  was  once  sea  mud,  while  metamorphism  has  gone  far 
enough  to  develop  the  garnets.  In  some  metamorphic 
rocks  the  planes  of  bedding  or  stratification  remain  to 
show  their  origin.  But  the  student  must  be  on  his  guard 
in  such  observations,  because  planes  of  cleavage  of  later 
origin  resemble  bedding  planes.  A  sheet  of  metamorphic 
rock,  showing  no  trace  of  former  fragmental  condition, 
may  lie  between  two  beds  of  limestone.  The  limestones 
are  presumably  sediments,  and  the  included  mass  is  likely 


COMPOSITION  AND  MINUTE  STRUCTURE   OF  ROCKS    211 

to  have  been  such.  One  of  the  best  proofs  of  metamor- 
phism  is  sometimes  at  hand :  namely,  to  find  the  sheet  of 
rock  shading  off  into  unmodified  sediments  as  it  is  traced 
for  some  distance. 

199.  Causes  of  metamorphism. — There  is  considerable 
agreement  as  to  the  general  agents  which  promote  interior 
changes  in  the  structure  and  constitution  of  rocks,  but 
much  remains  to  be  known  of  the  exact  nature  of  the  sev- 
eral processes,  and  of  the  way  in  which  they  interact  among 
themselves.  Thus  it  is  known  that  heat  greatly  favors 
metamorphism.  It  can  not  be  extreme  measures  of  heat, 
for  these  would  melt  the  rock,  which  might  not  then  be 
readily  distinguished  from  eruptives.  According  to  Dana, 
500°  to  1,200°  F.  is  heat  sufficient  for  the  work  done,  when 
taken  in  connection  with  other  agents  yet  to  be  named. 
The  heat  may  be  derived  in  some  measure  from  the  earth's 
interior,  but  is  more  largely  of  dynamic  origin.  Hence  it- 
is  that  regions  of  crushing  are  regions  also  of  metamor- 
phism. 

Moisture  also  hastens  metamorphism.  Especially  when 
heated  does  it  lessen  the  coherence  of  constituent  particles 
and  favor  their  rearrangement— that  is,  it  aids  in  making 
the  rock  plastic.  Much  water  is  not  needful.  We  have 
already  learned  that  all  rocks  contain  water.  Dana  shows 
that  the  average  amount  of  water  would  furnish  nearly  45 
cubic  feet  of  steam  for  each  cubic  foot  of  rock. 

Pressure  is  another  essential,  or  at  least  common  agent, 
in  metamorphism.  Such  pressures  are  mainly  due  to  crush- 
ing movements  of  the  earth's  crust,  though  something  is 
to  be  ascribed  to  the  weight  of  overlying  masses  of  rock. 
That  the  latter  may  not  be  enough,  however,  is  shown  by 
the  fact  that  sediments  once  buried  under  many  thousand 
feet  of  rocks  and  now  brought  to  light  by  denudation,  may 
not  show  metamorphism.  It  is  also  known  that  the  pres- 
ence of  alkaline  substances  greatly  hastens  some  metamor- 
phic  changes,  as  the  solution  of  silica,  which  is  hardly 


212  GEOLOGY 

affected  by  water  under  ordinary  conditions  at  the  surface 
of  the  earth. 

200.  Examples  of  metamorphic  rocks. — There  are  all  grades 
of  metamorphism,  shading  down  into  a  simple  consolidation 
of  sediments,  and  perhaps  extending,  on  the  other  hand,  to 
complete  melting.  Thus  some  granites  are  believed  not  to 
be  originally  igneous,  but  to  be  the  products  of  extreme 
metamorphism.  Hence  we  may  again  see  how  difficult  it 
is  to  classify  the  facts  of  Nature.  We  have  learned  that 
fragmental  rocks  may  be  the  debris  of  igneous,  metamor- 
phic, or  of  older  fragmental  rocks.  Metamorphic  rocks 
may  be  modified  sediments  or  modified  igneous  products, 
while  igneous  masses  may  be  parts  of  the  originally  molten 
globe,  or  result  from  the  melting  of  any  and  all  other 
classes  of  rocks.  Any  particle  of  matter  may,  during  the 
history  of  the  globe,  have  gone  through  many  such  changes. 
We  now  turn  to  the  principal  metamorphic  types. 

(1)  Marble  or  metamorphosed  limestone. — Ordinary  crys- 
talline limestone  which  will  take  a  good  polish  is  sometimes 
called  marble.     But  the  true  marble  is  composed  of  the 
crystalline,  granular  carbonate  of  lime.    That  it  is  a  changed 
limestone  is  shown  by  its  being  sometimes  associated  with 
fossils.     Geikie  cites  several  places  in  which  limestone  was 
changed  into  granular  marble  next  to  a  dike.     These  are 
cases  of  local  metamorphism  by  heat.     Chalk  and  litho- 
graphic limestone  have  been  changed  into  marble  artificially 
in  the  laboratory.     The  marbles  found  occupying  a  consid- 
erable area  or  belt,  as  in  Vermont,  are  an  illustration  of 
regional   metamorphism.     Pure   carbonate   of  lime  forms 
white  marbles,  but  other  substances  are  often  present,  giv- 
ing a  variety  of  colors,  as  seen  in  the  large  number  of  orna- 
mental marbles. 

(2)  Slate.— Shales  and  clayey  rocks  in  general  are  meta- 
morphosed into  compact,  hard  slates.     A  mud  rock  which 
would  break  down  under  a  winter's  frost  may  thus  be  made 
resistant  to  the  changes  of  many  years.     Such  is  the  origin 


COMPOSITION  AND  MINUTE  STRUCTURE  OF  ROCKS    213 

of  roofing  slates  and  those  of  school  use.  These  split  with 
great  facility  out  of  the  blocks  which  are  lifted  from  the 
quarry.  The  planes,  however,  are  planes  of  cleavage  (sec- 
tion 231),  while  the  original  planes  of  deposit  are  usually 
obliterated.  Hoofing  slates  occur  in  various  shades  of  blue, 
green,  brown,  and  red.  Other  slaty  rocks  do  not  thus 
cleave,  but  are  much  divided  by  joints.  The  slates  of 
Somerville  and  Braintree,  Mass.,  are  illustrations. 

(3)  Quartzite. — A  sandstone  composed  mainly  of  quartz 
grains  is  changed  by  metamorphism  into  a  compact  quartz- 
ite.     The  change  of  texture  is  commonly  due  both  to  pres- 
sure and  to  the  deposit  of  secondary  silica  among  the  sand 
grains.     It  thus  becomes  a  very  durable  rock. 

(4)  Gneiss. — This  is  a  banded  or  foliated  rock,  and  the 
more  common  or  typical  gneiss  has  the  same  chief  minerals 
as  granite.     Sometimes  the  bands  are  very  perfectly  devel- 
oped, thin,  and  allow  of  easy  splitting.     In  other  gneisses 
the  banding  is  coarse  or  obscure,  and  thus  there  is  a  grada- 
tion into  the  granite.     In  some  cases  this  rock  has  its  origin 
in  the  metamorphism  of  granite,  while  in  others  it  is  be- 
lieved to  be  derived  from  a  sedimentary  mass,  as  sandstone. 
Gneiss  is  abundant  among  the  pre-Paleozoic  formations. 

(5)  Other  schistose  rocks. — A  schist  is   a  rock  whose 
minerals  are  crystalline,  and  form  leaves  or  layers  along 
which  splitting  readily  takes  place.     Gneiss,  as  above  said, 
has  this  structure  in  greater  or  less  degree.     We  may  have 
a  quartz  schist  in  which  enough  scales  of  mica  are  present 
to  give  the  rock  a  foliated  character.     With  less  quartz  and 
more  mica  we  have  mica  schist,  common  among  the  meta- 
morphic  rocks  of  western  Xew  England.     It  shines  with 
scales  of  mica,  splits  easily,  and  readily  disintegrates,  filling 
with  its  bright  spangles  the  sand  which  is  produced.     Gar- 
net and  other  minerals  occur  as  accessory.     Other  schists 
take  names  from  minerals  which  are  prominent  in  the  com- 
position ;    thus   we  have    talc,   chlorite,  and    hornblende 

schists. 

15 


214  GEOLOGY 

(6)  Anthracite. — This  is  derived  by  metamorphic  pro- 
cesses from  soft  or  bituminous  coal.  Thus  the  anthracite 
of  Scranton  and  Wilkesbarre  is  of  the  same  age  as  the  soft 
coal  of  western  Pennsylvania,  Ohio,  and  Illinois,  but  has 
been  subject  to  powerful  compression  in  the  mountain- 
building  of  the  eastern  region.  Similarly  all  the  coals  of 
Colorado  are  soft,  save  in  a  single  region  of  volcanic  dis- 
turbance, where  anthracite  occurs.  In  harmony  with  these 
results,  graphite,  a  still  purer  form  of  carbon,  is  found 
among  very  ancient  and  powerfully  metamorphosed  rocks, 
as  in  Canada. 

The  student  should  remember  that  we  have  described 
but  a  few  examples  of  the  rocks  of  the  earth's  crust.  Their 
variety  is  infinite,  and  it  is  the  work  of  a  lifetime  to  know 
them.  But  it  is  also  true  that  faithful  study  of  the  rocks 
themselves  will  soon  give  practical  familiarity  with  the 
common  rocks  which  we  see  in  quarries,  and  in  the  various 
structures  which  man  makes  out  of  stone. 


CHAPTEE  XIII 

THE  GROSS  STRUCTURE  OF  ROCKS 

201.  THUS  far  we  have  studied  rocks  in  their  minute 
characters,  of  which  some  are  plain  to  the  unaided  eye, 
while  others  reveal  themselves  by  chemical  tests  or  by  the 
use  of  a  microscope.     "We  turn  now  to  a  series  of  larger 
structures.     We  begin   with   those   which   occur  only   or 
chiefly  in  sedimentary  rocks,  such  as  strata,  folds,  and  un- 
conformity.    Then  follow  structures  common  to  all  rocks, 
of  which  joints,  faults,  and  veins  are  examples.     Finally,  we 
come  to  certain  forms  and  structures  peculiar  to  igneous 
rocks,  such  as  dikes,  intrusive  sheets,  and  volcanic  necks. 
The  composition  and  minute  structure  of  rocks  are  related 
to  gross  structures  somewhat  as  organic  chemistry  and  his- 
tology are  related  to  gross  anatomy  in  the  study  of  plants 
and  animals. 

STEUCTUEE  OF  SEDIMENTABY  ROCKS 

202.  Stratification.— Sedimentary  rocks   are  always   de- 
posited in  more  or  less  distinct  layers,  and  are  said  to  be 
stratified.     This  is  the  most  important  rock  structure  with 
which  the  geologist  deals.     A  single  layer  is  called  a  bed, 
and  hence  the  sedimentary  rocks  are  often  called  bedded. 
A  succession  of  layers  of  the  same  kind  is  a  stratum,  which 
may  be  several  or  many  feet  thick.     The  term  stratum  is, 
however,  very  commonly  used  as  synonymous  with  bed. 

203.  Appearance  of  stratified  rocks. — If  one  visits  a  gravel 
or  sand  pit,  he  will  commonly  find  beds  of  sand,  or  some- 

215 


216  GEOLOGY 

times  of  fine  mud  alternating  with  layers  of  gravel,  or  vari- 
ations of  coarse  and  fine  sand.  The  finer  beds  are  deposited 
in  gentle  currents,  or  when  wave  action  is  moderate ;  the 
coarse  beds  when  such  movements  are  strong.  It  is  the 
difference  between  storm  and  calm,  floods  and  low  water, 
rainy  and  dry  periods.  When  we  observe  the  walls  of  a 
quarry  or  natural  ledge,  we  often  find  beds  and  division 
planes  without  such  changes  between  coarse  and  fine  mate- 
rials. Not  infrequently  a  thin  layer  of  shale,  or  fine,  con- 
solidated, argillaceous  mud  separates  the  beds,  as  of  lime- 
stone. Sometimes  the  division  is  determined  by  a  thin 
layer  of  fossils,  along  whose  plane  the  mass  splits  readily. 
Weathering  always  develops  the  bedding  planes,  so  that 
in  ledges  or  old  quarries  spaces  as  thick  as  one's  hand 
may  sometimes  be  found  between  layers.  The  similarity 
of  texture  of  several  beds  of  a  thick  stratum  is  due  to 
their  deposit  in  deep  or  quiet  waters,  where  variations 
of  velocity  could  not  occur.  The  origin  of  bedding  planes 
in  such  situations  is  not  so  clear.  They  may  some- 
times be  due  to  fossils,  as  above  stated,  and  sometimes 
to  periods  of  non-deposition,  during  which  partial  con- 
solidation of  the  last  laid  bed  or  of  its  upper  surface  takes 
place. 

Beds  are  of  variable  thickness,  from  one  or  a  few  inches 
to  several  feet.  In  the  latter  case  they  are  called  massive. 
A  bed  often  indicates,  by  a  fine  banding  on  its  edge,  many 
subordinate  layers,  which  are  called  laminae  or  leaves. 
Sometimes  the  rock  splits  freely  on  the  planes  of  lamina- 
tion, as  in  the  fine  paperlike  shales  of  Florissant,  Col. 
There  is  no  sharp  distinction  between  beds  and  laminse. 

204.  Position  of  stratified  rocks.— When  undisturbed,  this 
is  commonly  nearly  horizontal,  but  not  often  exactly  so. 
Such  rocks  are  made  on  gently  descending  lake  or  sea  bot- 
toms, or  along  the  slight  incline  of  a  river  valley.  The 
rocks  of  central  and  western  New  York  generally  dip  south- 
westward  or  southward  25  to  50  feet  per  mile.  They  may 


THE  GROSS  STRUCTURE  OF  ROCKS  217 

have  inclined  as  much  as  that  when  first  formed,  though 
slight  variations  have  doubtless  been  introduced  by  oscil- 
lations with  warping.  Taken  as  a  whole,  the  beds  of  a 
pond  or  lake  are  saucer  or  platter  shaped,  and  some  central 
portions  may  thus  be  horizontal.  Highly  inclined  but  un- 
disturbed beds  are  found  in  many  aqueo-glacial  deposits 
and  some  deltas. 

205.  Vertical  succession  of  strata. — This  may  be  shown 
on  the  sides  of  gorges  or  of  mountains  by  deep  borings,  or 
more  often  by  successive  outcrop  of  slightly  inclined  beds 
across  the  face  of  the  country.     It  is  common  for  lime- 
stones, sandstones,  and  shales  to  succeed  one  another  in 
various  order.     In   one  part  of  the   New  York  series  of 
rocks  a  sandstone   12  feet  thick  lies  between  two  lime- 
stones.    Above  the  upper  limestone  come  fine  black  shales, 
and  these  are  succeeded  by  a  great  alternation  of  sand- 
stones and  gritty  shales.     Sometimes  the  change  from  one 
kind  of  rock  to  the  next  is  abrupt,  but  often  it  is  gradual 
by  passage  beds  of  intermediate  character. 

206.  Horizontal  extension  of  strata, — Some  strata  extend 
for  hundreds  of  miles,  and  single  beds  may  be  traced  for 
considerable  distances.     But  change  of  thickness  and  char- 
acter generally  accompanies  such  horizontal  prolongation. 
Not  infrequently  a  bed  thins  out  in  one  or  more  directions, 
forming  a  wedge  or  a  lenslike  mass  between  others.     Upon 
a  little  reflection  the  student  will  see  that  this  is  the  form 
in  which  bodies  of  sediment  must  now  be  laid  down  in 
lakes  and  seas.     The  Niagara  limestone,  and  some  other 
formations  which  are  important,  in   western   New  York, 
thin  down  and  nearly  or  quite  disappear  south  of  the  Mo- 
hawk Kiver.     Sandstones  are  made  alongshore,  and  fine 
muds  in  the  deep  waters.     By  such  changes  in  ancient  strata 
the  place  of  shores  and  deep  waters  is  often  determined  for 
former  periods.     It  is  by  the  study  of  present  conditions 
that  we  may  read  the  geography  of  land  and  sea  in  far-dis- 
tant ages. 


THE  GROSS  STRUCTURE  OF  ROCKS 


219 


207.  Cross  bedding  or  false  bedding.— This  is  the  term 
used  when  the  laminae  of  a  bed  are  oblique  to  the  general 
planes  of  stratification.  Such  beds  are  formed  in  shallow 
water,  and  therefore  occur  most  often  in  sandstones.  Small 
embankments  are  formed  under  water  with  a  sloping  front 
advancing  with  successive  deposition,  like  a  railway  filling. 


FIG.  95. — Limestone,  showing  cross  bedding  and  columnar  structure, 
near  Cats-kill,  N.  Y. 

208.  Ripple  and  rill  marks. — The  former  may  be  observed 
on  almost  any  shallow  bottoms  where  the  waters  are  stirred 
by  the  wind.  They  may  be  covered  by  quiet  later  deposi- 
tion and  preserved.  Perfect  examples  are  thus  found  in 
the  most  ancient  sedimentary  formations.  When  the  waves 
retire  from  a  shelving  sand  beach,  little  rills  of  water  flow 
down  the  incline  and  erode  small  channels  in  the  sand. 
They  may  excavate  pockets  about  an  opposing  pebble  or 


220  GEOLOGY 

shell.     Such  structures  are  likewise  found  on  the  surface 
of  layers  of  ancient  sandstones. 


FIG.  96. — Ripple  marks  on  Triassic  sandstone.  Turner's  Falls,  Mass.     Slab  43  by  24 
inches,  of  which  one  half  is  here  shown.     Photograph  by  N.  Y.  State  Museum. 

209.  Rain  prints  and  sun  cracks.— Let  the  water  soak 
away  from  a  roadside  pool,  and  raindrops  splash  upon  the 
soft  surface  mud.     A  roundish  impression  is  made  which 
may  even  show  the  direction  in  which  the  rain  fell.     Or 
the  drying  of  the  mud  shrinks  and  cracks  it  into  rough 
polygonal  blocks.     Let  another  rain  ensue,  and  the  fresh 
supply  of  mud  will  fill  the  cracks  and  cover  the  bottom  of 
the  pool  anew,  thus  making  a  cast  or  mold  of  the  cracked 
layer.     Both  this  structure  and  the  rain  prints  are  some- 
times found  in  splitting  open  beds  of  ancient  rocks. 

210.  Fossils. — These    are   commonly    small    structures, 
sometimes  harder,  but  often  softer,  than  the  rock  which 


THE  GROSS  STRUCTURE  OF  ROCKS 


221 


holds  them.  In  the  latter  case  especially  they  are  a  source 
of  weakness,  and  favor  the  disintegration  of  the  mass. 
While  not  very  important  as  physical  structures  save  in 


FIG.  97.— Recent  rain  prints. 


FIG.  98. — Sandstone,  showing  ancient  mud  cracks,  Portland,  Conn. 
Photograph  by  W.  H.  C.  PYNCHON. 

their  contribution  to  the  limestones  of  the  world,  they  are 
of  supreme  importance  in  tracing  the  thread  of  the  earth's 
history. 


222 


GEOLOGY 


211.  Concretions  (from  con  and  crescere,  to  grow  togeth- 
er).— These  are  aggregates  of  some  mineral  lying  often 
in  a  sedimentary  bed  of  different  character,  as  nodules  of 


flint  in  limestone.  They  may  be  spherical,  or  round  and 
flattened,  or  elliptical,  or  of  irregular  and  fantastic  forms. 
By  the  inexperienced  they  are  often  taken  for  fossils  or 
artificially  fashioned  objects.  They  may  be  as  small  as  the 

head  of  a  pin,  or  several 
feet  in  diameter,  with  all 
intermediate  sizes.  Some 
of  the  most  curious  forms 
are  of  clay.  Flints  are 
found  in  many  limestones, 
as  in  the  chalks  of  Eng- 
land. Sometimes  they  form 
layers  interbedded  with  the 
limestone.  They  are  due 
to  the  solution  of  siliceous 
shells  and  sponge  skeletons 
and  the  aggregation  of  the 

FIG.  lOO.-Clay-ironstone  concretion,  Port-     Dissolved    matter.       So    also 
age  group,  shore  of  Lake  Erie.    Photo- 
graph by  N.  Y.  state  Museum.  carbonate  of  lime  may  form 

concretions,  as,  for  exam- 
ple, the  grains  of  oolitic  limestones,  which  have  been  found 
in  process  of  growth  in  modern  seas.  Similar  oolitic  con- 
cretions make  up  the  iron  ores  of  the  Clinton  epoch. 


THE  GROSS  STRUCTURE  OF  ROCKS  223 

The  nodules  are  often  formed  by  the  deposit  of  matter 
in  concentric  layers  about  some  object,  commonly  a  fossil. 
Elongated  concretions  containing  perfectly  preserved  ferns 
are  found  at  Mazon  Creek,  111.  Sometimes  concretions  in 
drying  develop  a  network  of  cracks  within.  These  cracks 
may  fill  with  other  material,  giving  the  mass  the  appearance 
of  a  turtle ;  hence  the  common  name  turtle-stones.  The 
geologist  calls  them  septaria.  Sometimes  they  are  3  or  4 
feet  in  diameter,  and  sections  of  them  polished  are  used  for 
ornamental  work.  Decomposing  organic  matter,  or  waters 


-^^^^^^  ^^^ 

FIG.  101.— Hand  specimen  of  crumpled  gneiss.     Photograph  by  G.  H.  WILLIAMS. 

bearing  a  cementing  substance,  may  by  infiltration  from  a 
center  bind  surrounding  particles  together,  as  the  grains  of 
a  sandstone,  making  a  kind  of  concretion  which  in  some 
cases  is  crossed  by  the  original  planes  of  deposit. 

If  a  cavity  formed  in  any  manner  becomes  lined  with 
crystals,  the  structure  is  called  a  geode.  It  agrees  with 
concretions  in  being  concentric. 

212.  Folds.* — These  are  chiefly  important  in  sedimentary 
rocks.  Unstratified  masses  might  be  folded,  but  they  are 
commonly  so  broken  and  disturbed  as  to  obscure  the  folds 

*  The  student  may  profitably  consult  also  the  text  and  the  illustra- 
tions in  the  section  on  mountains,  pp.  254-262. 


FIG.  102.— Folds  made  in  laboratory,  Willis.    The  block  thickens  and  the  crumpling 
increases  with  added  pressure. 


THE   GROSS  STRUCTURE  OP  ROCKS  225 

that  may  have  been  formed.  Rocks  are  folded  by  pressure 
following  the  direction  of  the  bedding  planes.  The  distor- 
tions vary  from  small  wrinkles  seen  in  a  hand  specimen  to 
folds  several  miles  in  height.  In  the  latter  case  a  territory 
some  scores  of  miles  wide  and  many  hundred  miles  long 
may  be  affected.  Folds  identical  in  appearance  have  been 
made  in  the  laboratory  by  Willis  and  by  Cadell  (Scotland). 
A  pile  of  thin  sheets  of  rocky  matter  was  artificially  made, 
put  under  a  heavy  load,  and  subjected  to  powerful  side 
thrust.  These  experiments  are  important  because  they 
help  us  to  understand  the  making  of  the  greatest  moun- 
tains. The  student  may  imitate  the  process  by  taking  a 
stack  of  sheets  of  paper  in  both  hands,  and  crumpling  them 
into  a  series  of  up-and-down  folds. 

A  fold  whose  bend  is  upward  is  called  anticlinal,  or  an 
anticline  (meaning,  inclining  in  opposite  directions).  A 
line  running  with  the  crest  is  the  axis,  and  the  rocks  on 
either  side  make  the  limbs  of  the  fold.  Folds  are  close  or 
open,  according  to  the  amount  of  force  used  in  their  mak- 
ing. The  Alps  illustrate  the  former  case,  the  Jura  and  the 
northern  Appalachians  the  latter.  Close  folds  rarely  stand 
upright,  but  tip  or  are  completely  overthrown.  Thus  the 
top  of  the  Jungfrau,  one  of  the  high  Alpine  summits,  is 
composed  of  most  ancient  crystalline  rocks,  surmounting, 
by  overturn,  sediments  of  vastly  younger  age. 

The  down-fold,  which  is  found  in  alternation  with  the 
up-f  old,  is  synclinal  or  a  syncline.  A  line  running  with  the 
trough  is  the  axis,  and  the  rocks  rising  on  either  side  are 
the  limbs  of  the  syncline.  One  of  these  inclines  forms  at 
once  the  limb  of  the  syncline  and  of  its  adjoining  anticline. 

It  must  not  be  thought  that  the  ridges  and  troughs  of 
such  folds  commonly  appear  as  surface  features.  They  are 
nearly  always  destroyed  by  erosion,  and  their  existence 
must  be  learned  in  other  ways.  Often  a  valley  follows  the 
anticline,  and  a  ridge  or  mountain  the  syncline.  Or  the 
upturned  edge  of  the  hardest  stratum  in  the  folded  series 


THE   GROSS  STRUCTURE  OP   ROCKS 


227 


stands  out,  as  the  Medina  sandstone,  for  example,  forms 
the  mountain  ridges  of  eastern  Pennsylvania. 


FIG.  104.— Sharply  contorted  and  greatly  denuded  strata.— From  LOGAN. 

As  some  folds  are  close  and  others  are  open,  others  still 
are  so  open  that  they  become  scarcely  more  than  gentle 
undulations  of  strata.  Such  faint  foldings  may  appear  on 
either  side  of  a  powerfully  disturbed  region,  representing 
the  fading  out  of  the  compressive  force. 

Monodinal  folds. — If  a  bed  or  stratum  passes  from  one 
horizontal  plane  to  another  by  means  of  a  bend  or  double 
curve,  the  fold  is  called  monoclinal,  or  a  monocline. 


FIG.  105.— Open  folds ;  valley  along  the  up-fold ;  ridges  along  the  down-folds. 

213.  Dip. — Kocks  disturbed  as  above  described  are  made 
to  incline  more  or  less  with  a  horizontal  plane.  The  angle 
of  inclination  is  the  dip.  The  term  is  usually  reserved  for 
beds  which  slope  by  reason  of  deformation,  while  we  speak 
of  the  inclination  of  beds  which  have  this  attitude  by  orig- 
inal deposit.  The  dip  varies  from  zero  to  90°,  when  the 
beds  become  vertical.  The  geologist  records  the  amount  of 
dip  and  its  direction,  as  IS.  20°  W.  or  E.  40°  S.  By  knowing 
the  dips  at  many  points  and  plotting  them  on  a  map,  the 
existence  and  extent  of  great  folds  can  be  determined,  or 
the  depth  of  certain  strata  at  a  given  point  be  made  out. 
Thus  a  geologist  may  be  able  to  find  the  depth  of  an  oil-  or 


228  GEOLOGY 

water-bearing  bed  or  of  a  coal  seam.     The  dip  is   deter- 
mined by  an  instrument  called  the  clinometer. 

Sf 


FIG.  106.— Section  of  inclined  strata.  To  find  the  thickness  b,  c,  we  solve  the  right- 
angled  triangle  a,  b,  c,  of  which  we  have  the  angle  a  (the  dip),  and  the  hypot- 
enuse a,  b. 

214.  Strike. — This  is  a  horizontal  line,  perpendicular  to 
the  dip,  and  may  be  straight  or  curved.     The  more  rap- 
idly the  direction  of  the  dip  changes  from  point  to  point  of 
a  given  bed,  the  more  curved  or  broken  is  the  line  of  the 
strike.     "  If  a  piece  of  slate  be  held  in  an  inclined  position 
and  lowered  into  a  vessel  of  water,  the  wet  line  will  repre- 
sent the  strike"  (Scott).     If  the  vessel  be  a  pan  with  flar- 
ing sides,  the  intersection   of  the   water  plane  with   the 
sides  will  illustrate  a  curved  strike. 

215.  Outcrop. — This  term  is  often  used  of  any  natural 
exposure  of  rock.     It  may  also  stand  for  the  belt  along 
which  a  given  stratum  would  be  exposed  if  there  were  no 
soil  cover. 

216.  Unconformity. — If  a  rock  mass  is  subject  to  erosion, 
and  the  newly  sculptured  surface  is  submerged  and  other 


FIG.  107. — Unconformity.     Here  the  history  recorded  is  :  deposit ;  uplift  without 
tilting  ;  denudation  ;  submergence  ;  deposit. 

strata  are  laid  down,  the  discordance  between  the  strata  is 
called  unconformity.  It  has  great  importance,  because  it 
shows  difference,  and  often  vast  difference,  in  age.  The 


THE   GROSS  STRUCTURE  OF  ROCKS 


229 


beds  below  the  unconformity  are  elevated  above  the  water 
level,  and  commonly  tilted  as  well  as  denuded,  before  the 
upper  beds  are  deposited.  Fig.  107  shows  elevation,  erosion, 
and  subsequent  deposit,  without  tilting.  Of  course  resub- 
mergence  must  follow  erosion,  to  admit  of  new  deposits. 


Fio.  108.— Unconformity.    Here  the  history  is  :   deposit  ;  uplift  with  tilting  ;   denu- 
dation ;  submergence  ;  deposit. 

In  Fig.  108  tilting  is  added  to  the  series  of  changes,  and  we 
can  see  how  much  of  history  is  revealed  by  a  single  sec- 
tion. Thus  a  series  of  beds  was  first  laid  down.  Later,  the 
beds  were  tilted  to  a  high  angle,  and  then  planed  down. 


FIG.  109.— Unconformity.  Conglomerate  on  quartzite,  Ouray  and  Silverton  Toll 
Road.  Col.  Dark  spot  due  to  shadow  of  overhanging  conglomerate.  Photograph 
by  the  author. 

Subsidence  followed  with  deposit  of  the  overlying  strata. 
The  student  must  not  expect  to  find  an  unconformity 
16 


230  GEOLOGY 

exposed  for  more  than  a  short  distance  in  a  region  covered 
with  soil  and  vegetation. 

The  break  in  continuity  of  deposition  may  be  short,  or 
it  may  comprehend  geological  eras.  Thus  if  beds  of  lake 
mud  rest  on  Algonkiau  limestone,  the  break  comprehends 
all  of  Paleozoic,  Mesozoic,  and  Tertiary  time.  The  red  sand- 
stones of  the  Connecticut  Valley  lie  unconformably  against 
the  crystalline  rocks  of  the  uplands.  Here  the  gap  is  vastly 
shorter. 

STRUCTURES  COMMON  TO  ALL  BOCKS 

These  fall  under  four  heads.  Joints  and  veins  are 
equally  important  in  the  several  kinds  of  rocks.  Faults 
may  occur  in  all,  but  are  more  conspicuous  and  more  readily 
detected  and  measured  in  the  bedded  rocks.  Cleavage 
may  affect  all  rocks,  but  in  the  case  of  sediments,  often 
nearly  or  quite  destroys  the  planes  of  bedding. 

217.  Joints. — In  sedimentary  rocks  there  are  commonly 
two  sets  of  dividing  planes,  nearly  perpendicular  to  the 
planes  of  bedding  and  to  each  other.  They  thus  roughly 
cut  the  mass  into  rectangular  blocks.  Almost  any  cliff 
shows  this,  and  the  work  of  quarrying  is  thus  greatly  aided. 
Often  one  system  of  joints  is  more  perfect  than  the  other, 
and  may  show  on  exposure  a  very  perfect  and  smooth  wall 
face.  The  effect  of  joints  on  erosion  has  been  noticed  in 
Part  I.  The  frequency  of  joints  in  bedded  rocks  is  vari- 
able. Sometimes  they  crowd  one  another  at  intervals  of 
an  inch  or  less.  At  other  times  they  are  one,  two,  or  more 
feet  apart,  up  to  ten  or  twenty,  as  seen  in  some  very  large 
flagstones.  Joints  are  common  in  igneous  rocks,  as  granite 
and  basalt.  The  peculiar  columnar  jointing  of  lavas  will 
be  described  under  the  structures  peculiar  to  igneous  rocks. 

In  regions  of  much  disturbance  the  joints  may  be  very 
perfect,  and  be  found  cutting  one  another  at  various  angles 
and  in  several  systems,  even  as  many  as  six  or  seven.  The 
slates  of  Somerville,  Mass.,  show  this  well.  Joints  are  not 


THE  GROSS  STRUCTURE  OF  ROCKS 


231 


confined  to  the  more  ancient,  or  even  the  fully  consolidated 
rocks.  They  are  sometimes  well  developed  in  clays  which 
have  been  subject  to  drying  and  to  little  pressure. 

218.  Origin  of  joints.— Some  joints  may  be  due  to  shrink- 
age in  drying.  This  and  the  presence  of  elaborate  jointing 
in  some  slates  point  to  dynamic  pressure  as  the  important 
cause.  All  parts  of  the  earth's  crust  must  be  subject  to 


Fio.  110.— Stream  bed,  Ansable  River,  showing  two  systems  of  vertical  joints. 

strong  pressure,  even  where  there  is  no  metamorphism. 
Similar  results  have  been  produced  by  experiments  with 
blocks  of  ice.  An  account  of  this  may  be  found  in  Dana's 
Manual  of  Geology,  p.  372.  It  is  probable  also  that  shocks 
of  earthquakes  are  effective  in  making  joint  structure. 


232  GEOLOGY 

219.  Faults. — These  are  dislocations  in  which  a  mass 
of  rock  moves  on  the  adjacent  mass  along  a  plane  of  divi- 
sion, commonly  a  fracture.  More  often  the  movement  is 
vertical,  or  at  a  moderate  angle  from  the  vertical,  and  car- 


Fio.  111.— Normal  faults  ;  escarpments  not  removed  by  denudation. 

ries  sedimentary  strata  out  of  correspondence  with  each 
other.  Faulting  usually  goes  with  powerful  folding,  pro- 
ducing most  complicated  arrangements  of  the  rocks  con- 
cerned, especially  when  obscured  by  erosion  and  surface 
deposits.  The  amount  of  dislocation  is  called  the  throw, 
and  varies  from  a  fraction  of  an  inch  to  many  thousand 
feet.  The  limit  is  the  depth  at  which  all  rocks  become 
plastic  with  pressure  and  heat.  Faults  may  extend  for 
scores  or  hundreds  of  miles.  Toward  the  end  the  throw 
diminishes  and  the  dislocation  runs  out.  A  region  may  be 
broken  into  stupendous  crust  blocks  by  crossing  systems  of 
faults.  Mountain  ridges  and  intervening  lake  basins  may 
be  due  to  faults,  as  in  the  Great  Basin. 


FIG.  112.— Normal  faults  ;  escarpments  worn  away.     Broken  lines  show  outline  of 
country  if  there  had  been  no  denudation. 

It  must  not  be  supposed  that  the  rising  wall  of  a  fault 
generally  forms  a  cliff.  Often  no  sign  of  such  dislocation 
appears  in  the  topography.  Either  the  faulting  goes  on 
so  slowly  that  the  cliff  can  not  develop,  or  is  of  such 


THE  GROSS  STRUCTURE  OF  ROCKS  233 

ancient  date  that  it  has  been  destroyed  by  erosion.  In 
the  case  of  recent  faults  the  escarpment  sometimes  ap- 
pears. 

220.  Examples  of  faults.— As  already  cited,  the  moun- 
tains of  the  Great  Basin  region  are  vast  tilted  fault  blocks. 


FIG.  113.— Banded  sandstone,  Dakota,  showing  faulting. 

Lesley  describes  a  fault  of  8,000  feet  throw  in  southern 
Pennsylvania  so  cleanly  formed  that  one  might  plant  his 
feet  on  both  sides  of  the  plane  of  movement.  Dislocations 
of  2,000  feet  are  described  as  occurring  along  the  Appa- 
lachians. A  series  of  north  and  south  faults  have  brought 
up  across  the  Mohawk  Valley  ancient  gneisses,  which  would 
otherwise  be  buried  several  hundred  feet  below  the  river. 


234  GEOLOGY 

Along  the  Pennine  Chain  in  Yorkshire,  England,  is  a  fault 
fifty  miles  long  with  a  throw  of  4,000  feet. 


FIG.  114.— Fault  in  Gering  sands,  south  of  Crawford,  Neb. 
Photograph,  1897,  by  N.  H.  DARTON. 

221.  The  hade  of  a  fault  is  the  angle  which  its  plane 
makes  with  a  vertical.  The  overhanging  face  is  called  the 
hanging  wall,  while  the  other  face  is  called  the  foot  .wall. 
If  the  hanging  wall  goes  down  relatively  to  the  foot  wall, 
we  have  a  normal  fault.  It  is  such  as  would  be  produced 
by  tension,  causing  a  spreading  of  the  masses  concerned,  or 
it  might  be  due  to  gravity  alone,  if  support  were  weakened 
below.  If,  however,  the  hanging  wall  goes  up  relatively  to 
the  foot  wall,  we  have  a  reversed  fault.  It  is  such  a  move- 
ment as  would  be  caused  by  thrusting  together  masses  on 


THE  GROSS  STRUCTURE   OF  ROCKS 


235 


two  sides  of  an  inclined  plane  of  division.  Both  thrusts 
and  pulls  may  well  be  incidental  to  the  general  shrinkage 
of  the  earth's  crust. 

A  fold  may  by  extreme  pressure  pass  into  a  fault.  The 
thrust  may  then  be  so  great  as  to  carry  one  mass  of  strata 
several  miles  over  upon  another.  This  is  called  an  over- 
thrust  fault.  An  overthrust  of  11  miles  occurs  in  the  beds 
of  eastern  Tennessee.  Similar  gigantic  thrusts  have  oc- 
curred among  the  rocks  of  the  Scottish  Highlands.  A 
monoclinal  fold  may  pass  into  a  fault,  as  seen  in  Fig. 
115.  A  dislocation  of  this  sort  may  take  place  along 
several  planes  by  a  series  of  faults  of  small  throw.  Thus 


FIG.  115.— Diagrams  showing  successive  stages  (A,  B,  C)  in  the  making  of  a  reversed 
or  thrust  fault. 


we  have  a  step  fault.  Along  the  plane  of  faulting,  instead 
of  clean  faces  there  may  be  a  zone  of  crushing  several  or 
many  feet  thick.  Kock  surfaces  powerfully  moving  on 
each  other  form  highly  glazed  surfaces,  known  as  slicken- 


230 


GEOLOGY 


The  same  bed  may,  through  faulting,  appear  repeatedly 
at  the  surface,  as  in  Fig.  116.  This  principle  is  of  great  im- 
portance in  mining.  Suppose  c,  c,  c  in  Fig.  116  to  be  beds 


'I- 


FIG.  116.— Repetition  of  strata  by  faulting. 

of  coal.  False  expectations  would  be  raised  if  two  of  these 
were  not  ascertained  to  be  but  small  sections  of  the  one 
original  seam. 

VEINS 

222.  Definition. — A  vein  is  a  sheet  or  mass  of  one  or 
more  minerals  formed  by  the  slow  filling  of  a  fissure  or 
cavity,  or  by  replacement  of  more  or  less  of  the  original 
substance  of  the  rock.     The  term  is  often  popularly  used 
where  seam  or  bed  should  be  employed,  as  of  deposits  of 
coal,  salt,  or  iron  ore.     Veins  naturally  occur  most  fre- 
quently in  regions  of  disturbance,  where  the  subterranean 
geological  processes  are  active.     They  are  the  most  prolific 
source  of  the  more  valuable  metals,  of  rare  minerals,  and 
of  gems. 

223.  Origin  of  cavities. — This  may  be  due  to  shrinkage, 
as  in  drying  or  cooling.     Often  a  thin  vein  deposit  is  made 
along  joint  planes,  where  no  special  dynamic  activity  has 
occurred.     Other  cavities  or  pockets  are  formed  by  solu- 
tion, and  may  be  filled  with  vein  material  if  small,  or  may 
enlarge  into  caverns.    Certain  ores  of  lead,  as  at  Galena,  111., 


THE  GROSS  STRUCTURE  OF  ROCKS  237 

lie  in  solution  pockets  in  limestone.  By  far  the  greater 
number  of  veins  are  formed  in  fissures,  due  either  to  fault- 
ing, folding,  or  earthquake  shocks.  Fissure  veins  vary  in 
thickness  from  a  fraction  of  an  inch  to  several  or  many 
feet. 

224:.  Modes  of  filling.— This  process  is  believed  to  be 
accomplished  in  several  ways,  (a)  By  lateral  secretion. 
Particles  of  mineral  are  dissolved  and  carried  out  from  the 
rocks  and  deposited  in  adjoining  crevices.  The  character 
of  the  vein  would  depend  upon  the  composition  of  the 
local  rock,  (b)  By  descension.  Waters  flowing  down  from 
above  may  deposit  dissolved  matters.  Thus  stalactite  for- 
mations are  closely  related  to  such  veins,  and  might  be 
traced  up  into  veins  of  this  nature,  (c)  By  ascension. 
Heated  waters  come  from  below,  depositing  on  their  way 


FIG.  117.— Veins  of  calcite,  Highgate  Springs,  Vt.     U.  S.  Geological  Survey. 

the  vein  materials.  The  incrusting  of  pump  and  boiler 
pipes  is  an  illustration  of  such  effects.  Similarly  a  fissure 
may  be  filled  through  the  medium  of  ascending  vapors, 
whose  mineral  matters  are  sublimated  as  they  rise,  through 


238 


GEOLOGY 


loss  of  the  heat  by  which  they  were  brought  to  a  gaseous 
condition. 

225.  Veins  without  fissures.— Eeplacement  of  some  min- 
erals by  others  may  go  on  by  means  of  percolating  waters. 
We  may  have  a  rearrangement  of  minerals  already  present, 
or  an  importation  of  others  from  a  distance.     Such  mineral 
masses  are  not  typical  veins,  as  their  boundaries  are  ob- 
scure and  their  forms  quite  indefinite. 

226.  Further  facts    about    veins.— Some    veins   show   a 
banded  structure.     A  mineral  may  be  deposited  in  thin 
sheets  on  either  side  of  a  fissure.     These  sheets  may  then 
be  overspread  by  others  due  to  waters  or  gases  carrying  a 
different  substance.     The  banding  may  be  due  to  a  filling 


FIG.  118.— Vein  structure,  showing 
banding. 


FIG.  119.— Vein  structure.    Banding  and 
combs  of  interlocking  crystals. 


of  the  fissure  and  reopening,  giving  opportunity  for  another 
sheet  of  mineral  to  form  alongside  the  first.  Sometimes 
the  vein  carries  off  a  mass  of  the  rock  wall,  which  is  then 
closed  in  on  the  rent  face  by  vein  matter.  Such  an  iso- 
lated mass  is  called  a  "  horse."  Some  veins  show  comblike 
structure,  due  to  the  formation  of  interlocking  layers  of 
crystals  on  both  sides  of  the  fissure.  Miners  often  find 
a  clayey  selvage  between  a  vein  and  the  country  rock, 
due  to  grinding  or  to  decomposition. 

227.  Veins  and  metallic  deposits.— If  a  metal  is  not  in 
composition  or  chemical  union  with  some  other  substance, 
it  is  called  "  native,"  or  "  free."  Gold  almost  always  occurs 


THE  GROSS  STRUCTURE   OF  ROCKS 


239 


in  this  way,  and  silver  not  infrequently.    Thus  also  the  cop- 
per of  the  great  mines  of  northern  Michigan  is  free.     An 
ore    is    strictly    a    metal 
united  with  a  non-metallic    r 
substance.       The   term  is 
loosely  employed,  however, 
and  is,  for  example,  used 
for  quartz  which  contains 
gold.      The   gold  is  free, 
though  generally  so  finely 
disseminated  as  to  be  in- 
visible.   Many  ores  contain 
several  metals.      Much  of 
the  silver  ore  of  the  West 

yields    a    large    quantity  of    FIG.  120.— Vein  structure,  showing  horse,  H. 

lead  with   small    amounts 

of  gold  and  copper.     Ore  is  an  economic  term,  not  being 

used  unless  a  metal  is  present  in  workable  quantity. 

The  most  important  ores  of  precious  metals  are  found 
in  great  fissure  veins.  These  are  apt  to  be  parallel  in  a 
given  region,  and  go  far  down,  as  in  the  famous  Comstock 
mine  in  Xevada,  which  is  closed  to  development  below 
a  certain  level  by  the  abundance  of  heated  waters.  We 
should  expect  great  fissures  to  furnish  metalliferous  veins 
because  they  go  deep,  furnish  a  highway  for  subterranean 
waters,  and  are  themselves  attendants  of  disturbance  and 
consequent  metamorphic  action.  Parts  of  such  veins  may 
be  rich  and  others  of  no  value.  A  columnar  body  of  ore  in 
a  vein  is  a  "  chimney  "  or  "  chute."  A  great  pocket  of  pre- 
cious ore  is  a  "bonanza."  The  vein  stone  or  matrix  is  the 
"  gangue,"  and  fragments  of  a  vein  broken  off  at  the  sur- 
face are  called  by  the  prospector  "  float "  or  "  blossom." 

228.  Bedded  ore  deposits. — Such  are  most  ores  of  iron, 
the  lead  already  mentioned,  and  some  silver  ores,  as  at 
Leadville  and  Aspen,  Col.  By  old  mining  law,  a  mining 
claim  follows  the  vein,  even  though  its  angle  with  the  hori- 


240 


GEOLOGY 


zon  carries  it  below  an  adjacent  area.  The  evident  injus- 
tice of  this  in  wide-sweeping  bedded  deposits  led  to  an 
interesting  change  in  the  law,  limiting  the  horizontal  ex- 
tension of  claims. 

229.  High-  and  low-grade  ores. — These  are  relative  terms, 
and  the  usefulness  of  a  low-grade  ore  depends  upon  the 
perfection  of  the  processes  of  reduction.  With  the  prog- 
ress of  invention,  ores  are  now  worked  which  were  formerly 
cast  upon  the  dump  heap.  The  more  valuable  metals,  such 
as  gold  and  silver,  are  reckoned  in  ounces  per  ton ;  the 
less  valuable,  as  lead,  in  percentages  of  the  whole.  Keduc- 
tion  employs  various  processes  of  crushing,  concentration, 
roasting,  and  smelting,  which  can  not  be  explained  here. 


N  s 

FIG.  121.— Table  Mountain,  California.  Broken  line  shows  profile  of  ancient  moun- 
tains. 6,  lava  filling  of  old  valley  ;  li,  gravels  of  this  ancient  river  bed  ;  K',  R', 
present  river  beds. 

230.  Placers.— A  placer  is  a  deposit  of  gravel  containing 
particles  of  free  gold,  or  of  tin,  platinum,  or  other  ores. 
The  gravels  may  be  of  alluvial  or  glacial  origin,  and  they 
are  derived  from  the  wear  of  rocks  with  metalliferous  veins. 
The  gravels  may  belong  to  an  ancient  stream,  as  at  Table 
Mountain,  California,  where  a  valley  was  filled  with  a  lava 
stream,  and  the  gravels  since  made  accessible  by  denuda- 
tion are  found  to  yield  gold  (Fig.  121).  The  gold  dust  and 
nuggets,  being  heavy,  are  found  more  abundantly  at  the 
bottom  of  the  gravels.  Placer  beds  may  be  found  under 
the  borders  of  the  sea,  as  at  Cape  Kome,  Alaska.  "  Stream 
tin"  occurs  abundantly  in  Australia,  and  to  a  lesser  ex- 
tent in  California  and  other  parts  of  the  West.  We  have 
here  a  natural  process  of  crushing  and  partial  concentra- 


THE  GROSS  STRUCTURE  OF  ROCKS  241 

tion.  Various  methods  are  used  to  finish  the  work  of  con- 
centration. Among  these  are  the  primitive  pan,  with 
which  the  miner  agitates  the  gravel  in  water,  and  the 
"  cradle,"  a  small  sluice  with  "  riffles  "  (crossbars)  which 
catch  the  heavy  gold  as  a  gravel-bearing  current  of  water  is 
sent  over  them.  Hydraulic  mining  proceeds  on  the  same 
principle,  but  on  a  large  scale.  A  powerful  current  of 
water  is  sent  against  the  gravel  bank,  whose  materials  are 
carried  through  a  large  sluice  with  riffles.  The  flooding  of 
good  fields  by  these  gravel-bearing  streams  became  so  disas- 
trous in  California  as  to  require  regulation  by  law. 

231.  Cleavage. — We  have  already  studied  several  sorts  of 
divisional  planes,  such  as  those  of  bedding,  joints,  and 
fracture  with  or  without  faulting.  Cleavage  is  found  espe- 
cially among  metamorphic  rocks,  and  by  virtue  of  it  the 
mass  may  split  into  thin  leaves.  The  structure  is  typically 
developed  in  slates,  and  hence  is  sometimes  called  slaty 
cleavage.  It  differs  from  foliated  structure  in  the  perfec- 
tion with  which  rocks  affected  by  it  split  into  broad  thin 
sheets,  and  also  in  the  evident  appearance  in  foliated  rocks 
of  the  flakes  or  scales  of  the  minerals.  Cleavage  passes 
across  the  planes  of  bedding,  and  nearly  or  quite  obliterates 
them. 

Pressure  is  believed  to  be  the  cause.  The  cleavage 
planes  are  found  to  be  at  right  angles  to  the  direction  of 
the  force,  as  may  be  clearly  seen  in  mountain  masses.  Con- 
stituent grains  in  the  rock  may  be  found  flattened  and 
elongated,  and  fossils  that  survive  are  apt  to  be  distorted 
from  their  proper  forms.  Cleavage  has  been  artificially  in- 
duced, as  in  pipe  clay  mixed  with  scales  of  iron  oxide,  or  in 
beeswax. 

GENERAL  STRUCTURE  OR  MODE  OF  OCCURRENCE  OF 
IGNEOUS  KOCKS 

Melted  rocks  have  solidified  on  the  surface  at  moderate 
depths,  and  in  deeply  buried  parts  of  the  earth's  crust.  In 


THE   GROSS  STRUCTURE  OF  ROCKS 


243 


the  last  case  we  are  able  to  observe  them  only  because  they 
have  been  brought  to  light  by  extensive  denudation.  Igne- 
ous rocks  formed  at  the  surface  are  lavas,  and  having 
cooled  quickly,  as  compared  with  those  which  are  deep- 
seated,  are  less  crystalline  and  often  vesicular. 


FIG.  123.— Diagram  showing  dike  and  intrusive  sheets,  one  of  which  breaks  across 
the  strata. 

232.  Volcanic  cones  and  necks. — Cones  have  already  been 
described  in  our  account  of  the  workings  of  volcanoes.  If 
we  could  remove  the  cone  of  an  extinct  vent  and  strip 
away  its  foundations,  we  should  doubtless  find  a  column  or 
plug  of  igneous  rock  extending  downward  to  the  deep 


FIG.  124.— Laccolith  (after  GILBERT).    This  ideal  section  is  drawn  as  if  no  denuda- 
tion had  taken  place. 

sources  of  the  lava.  Xature  has  done  this  in  some  cases 
and  left  the  eroded  stump  projecting  above  the  surface, 
owing  to  its  greater  hardness.  Such  a  stump  is  called  a 


244 


GEOLOGY 


volcanic  neck.     Many  are  found  in  New  Mexico  .and  other 
parts  of  the  West. 

233.  Sheets. — In  a  number  of  regions  in  later  geological 
times  large  areas  have  been  covered  with  outpourings  of 
molten  rock  without  the  formation  of  cones  and  without 
evidence  of  a  single  or  central  vent.  The  discovery  of 
great  dikes,  or  lava  fillings  of  ancient  fissures,  goes  far  to 
prove  that  these  sheets  are  due  to  eruptions  through  cracks 
in  the  earth's  crust.  Such  accumulations  occur  in  Colorado, 
Utah,  Idaho,  Oregon,  Washington,  and  other  Western  States ; 
also  on  the  west  coast  of  Scotland,  in  Abyssinia  and  India. 


FIG.  125.— Granite  dike  cutting  crystalline  limestone,  192d  street,  New  York  City. 
Photograph  by  N.  Y.  State  Museum. 

Other  sheets  have  been  intruded  between  beds  of  sedi- 
ment, and  their  surfaces  or  edges  since  exposed  by  erosion. 
Sometimes,  as  in  the  lava  sheets  of  the  Connecticut  Valley, 
and  those  that  form  the  Palisades  of  the  Hudson,  the 
igneous  rocks  have  suffered  tilting  or  other  deformation 
along  with  the  sediments  which  inclose  them.  If  a  lava 
flows  between  two  layers  of  sediment,  the  latter  will  be 


THE  GROSS  STRUCTURE  OP  ROCKS  245 

baked  by  the  heat  both  above  and  below.     If  a  sheet  of 
lava  has  in  some  ancient  time  overflowed  the  surface,  and 


FIG.  126.— Dike,  Avalanche  Lake,  New  York.    The  dike  is  worn  away  more  than 
the  adjacent  rock,  and  a  waste  elope  is  built  into  the  lake. 

(Copyright,  1888,  by  S.  R.  STODDARD,  Glens  Falln,  N.  Y.) 

after  cooling  has  been  covered  with  a  deposit  of  sedimen- 
tary rock,  the  under  surface  of  the  latter  will  not  show 
17 


246 


GEOLOGY 


change.  Sometimes  the  lava  has  left  the  plane  on  which 
it  was  flowing  and  torn  its  way  across  other  beds,  to  resume 
there  a  direction  parallel  to  the  first  (Figs.  122  and  123). 
The  hanging  hills  near  Meriden,  Mount  Holyoke,  and  Mount 
Tom,  the  Palisades  of  the  Hudson,  the  Watchung  Mountains 
near  Orange,  N.  J.,  and  the  Salisbury  Crags  in  Edinburgh, 
furnish  well-known  examples  of  sheets.  In  all  these  cases 
the  beds  have  been  tilted  and  subjected  to  vast  erosion.  All 
gradations  may  occur  between  wide  sheets  of  nearly  uni- 
form thickness  and  the  structures  next  to  be  described. 

234.  Laccoliths  or  laccolites. — It  has  been  found  that  some 
masses  of  lava,  issuing  from  the  seats  of  volcanic  energy, 
did  not  reach  the  surface  of  the  earth,  but,  having  broken 
through  part  of  the  overlying  rock,  heaved  the  still  un- 
broken layers  up  into  great  domes  (Fig.  124.)  If  surface  ero- 
sion had  not  taken  place  on  these  domes,  it  would  be  nearly 
or  quite  impossible  to  show  their  origin.  But  they  have  been 
so  deeply  dissected  that  the  underlying  lavas  are  brought 
to  light,  still  bearing  sloping  remnants  of  the  uplifted  sedi- 
mentary beds.  This  kind  of  volcanic  structure  was  first 
adequately  studied  by  Mr.  G.  K.  Gilbert,  of  the  United 
States  Geological  Survey.  He  found  that  the  Henry 
Mountains  of  Utah  are  of  this  sort,  and  applied  the  term 
laccolite  (stone  cistern)  to  such  masses  of  lava.  Some 
mountains  of  western  Colorado  belong  to  the  same  class. 


FIG.  127.— Topographic  effects  of  hard  and  soft  dikes  intersecting  sedimentary  beds. 

235.  Dikes.— A  dike  is  like  most  veins  in  form,  and,  like 
a  vein,  is  found  along  fractures  or  other  planes  of  division. 
The  fissure  is,  however,  filled  rapidly  by  a  flow  of  molten 


THE  GROSS  STRUCTURE  OF  ROCKS 


24' 


rock,  rather  than  slowly  by  rock  matter  from  solution. 
The  dike  is  a  casting  run  into  a  mold.  Dikes  are  most 
variable  in  thickness,  ranging  from  less  than  an  inch  to 
many  feet.  Fifty  dikes  from  1  to  75  feet  wide  and  13  miles 


FIG.    128.— Dike,  Lake   Superior  (after   OWEN);    adjacent  rocks   weathered   awiiy. 
Note  the  horizontal  columns  and  compare  with  Fingal's  Cave,  Figr.  129.    • 

long  are  said  to  occur  in  the  Triassic  sandstones  of  North 
Carolina.  The  rock  in  a  dike  of  any  width,  is  usually  more 
coarsely  crystalline  in  the  central  portions,  where  cooling 
is  slow,  but  finely  crystalline  or  glassy  near  the  walls,  where 
cooling  is  rapid.  Lines  of  flowage  are  often  found,  and 
these,  with  the  varying  crystalline  characters  of  the  dif- 
ferent parts,  are  among  the  tests  by  which  a  dike  may  be 
distinguished  from  a  vein.  Local  metamorphism  may  be 
found  in  the  bounding  rocks  also.  If  the  rock  be  hard  or 
soft,  we  may  have  a  trench  or  a  wall  where  the  dike  comes 
to  the  surface  (Fig.  127).  Dikes  may  thus  form  natu- 
rally fenced  fields,  as  cited  for  one  place  in  Scotland ;  or 
we  may  find  a  line  of  bowlders,  or  vegetation  or  fresher 
green  over  the  dike,  to  mark  its  presence.  Dikes  are 


THE  GROSS  STRUCTURE  OF  ROCKS  249 

among  the  most  common  structures,  which  students  in 
some  regions  will  see  in  their  field  excursions.  Professor 
Shaler  estimates  5  to  10  per  cent  of  the  surface  of  Cape 
Ann  as  occupied  by  them.  Dikes  of  different  ages  often 
intersect  each  other,  the  later  ones  breaking  continuously 
across  the  earlier.  They  are  often  offshoots  of  volcanic 
necks,  bosses,  or  sheets,  and  cones  are  often  ribbed  with 
them. 

236.  Columnar  structure  of  lavas. — This  occurs  in  marked 
perfection  and  is  a  form  of  jointing.  The  columns  are 
polygons  in  form,  from  a  few  inches  to  several  feet  in  diam- 
eter, and  are  seen  in  especial  perfection  in  many  sheets  of 
basalt,  as  at  Fingal's  Cave  and  the  Giant's  Causeway.  The 
entire  island  of  Staffa,  several  adjacent  small  islands,  and 
the  shores  of  Mull  illustrate  this  structure  on  a  large  scale, 
as  also  the  Palisades  of  the  Hudson,  and  Obsidian  Cliff  in 
Yellowstone  Xational  Park.  The  axis  of  the  columns  is 
always  perpendicular  to  the  plane  of  the  sheet  or  dike. 
Sometimes  each  column  is  divided  into  sections  by  a  kind 
of  ball-and-socket  joint  whose  origin  is  obscure.  Xor  is  the 
columnar  jointing  well  understood,  but  is  believed  to  be 
due  to  contraction  in  cooling. 


PIG.  130.— Basaltic  column,  showing  ball-and-socket  joints. 


CHAPTEK  XIV 

PHYSIOGRAPHIC   STRUCTURES 

HAVING  studied  the  minute  and  gross  structures  of 
rocks,  we  must  now  see  how  these,  taken  together  and 
acted  on  by  geological  forces,  make  up  the  larger  elements 
of  the  earth's  crust.  We  shall  notice  two  or  three  great 
principles,  review  the  chief  kinds  of  physiographic  form, 
such  as  plains,  mountains,  valleys,  and  shore  lines,  seeing 
finally  how  these  develop  together  and  lend  form  to  con- 
tinent and  ocean  basin,  and  thus  give  to  the  globe  its 
surface  expression. 

237.  Definition. — Physiography  means,  according  to  its 
derivation,  a  description  of  Nature.     It  is  so  used  by  some 
English  authors.     In  America  it  is  sometimes  applied  to 
physical  geography  as  a  whole,  but  more  commonly  to  the 
science  of  land  forms.     We  here  use  the  word  in  the  latter 
sense.     It  is  the  study  of  land  forms,  including  the  topog- 
raphy of  ocean  basins,  in  the  light  of  their  origin,  or  from 
the  point  of  view  of  geology. 

238.  Land  mass  and  land  sculpture. — We  may  profitably 
think  of  the  exposed  parts  of  the  earth's  crust  as  a  block 
upon  which  erosive  forces  work,  much  as  the  artist  uses 
his  tools  upon  a   piece  of  marble  or  granite.     We  have 
seen  that  the  tools  for  earth  sculpture  are  various,  and 
work  in  many  ways.     Consider  a  given  mass  of  land.     The 
atmosphere  covers  it ;  its  oxygen  enters  into  combination 
everywhere,  and  the  winds  tear  its  surface,  modify  its  rain- 
fall, and  send  waves  upon  its  shores.     Streams  form  and 

250 


PHYSIOGRAPHIC  STRUCTURES  251 

run  over  it,  and  glaciers  destroy  or  build  upon  its  surface. 
It  rises  and  sinks  relatively  to  sea  level,  it  is  shaken  by 
earthquakes,  folded  and  broken  by  compressive  forces, 
dotted  with  volcanic  cones,  or  flooded  with  broad  streams 
of  molten  rock.  Meantime  life  flourishes,  and  the  chang- 
ing panorama  of  form  and  color  moves  before  the  eye,  or  is 
revealed  to  the  diligent  student  of  the  earth's  history. 

239.  Land  sculpture  and  rock  structures. — Land  form  de- 
pends not  more  on   geological  forces  than  it  does  upon 
the  rock  structures  studied  in  Chapters  XII  and  XIII. 
Whether  the  rock  is  hard   or  soft,  soluble  or  insoluble, 
crumbles  easily  or  holds  together  firmly — all  these  condi- 
ditions  influence  largely  the  effectiveness  of  erosion.     So, 
too,  the  large  structures  are  no  less  important.     Let  us 
take,  for  example,  the  topographic  effects  of  stratification. 
Hard  and  soft  beds  often  alternate  with  each  other.     If 
they  lie  horizontally,  a  hard  bed  which  can  not  easily  be 
destroyed  will  at  length  form  the  top  of  a  plateau  or  tabular 
hill.     Or  it  determines  the  existence  of  a  waterfall,  or  makes 
a  retreating  wall  of  rock  precipitous  instead  of  sloping. 

Folds  give  character  to  all  the  great  mountains  of  the 
earth.  In  every  variety  of  strength,  size,  and  combination, 
folds  carved  by  geological  implements  give  us  the  scenery 
of  the  Appalachians,  the  Cordilleras,  and  the  Alps.  Pow- 
erful folding  is  combined  with  faulting,  and  in  some  cases 
faulting  is  the  main  feature,  and  along  with  these  struc- 
tures go  those  of  the  igneous  rocks,  the  dike,  sheet,  lacco- 
lith, and  volcanic  neck,  in  every  degree  and  kind  of  asso- 
ciation. 

240.  Land  modeling. — Here  we  may  include  topographic 
changes  and  forms  which  might  appear  under  the  head  of 
land  sculpture.     But  as  they  are  made  by  deposit  rather 
than  erosion,  the  term  modeling  is  more  suitable.     Many 
forms  of  land  waste,  made  by  rivers,  glaciers,  winds,  or 
waves,  belong  here.     Taken  all  in  all,  therefore,  the  forms 
of  the  lands  are  very  complex  in   their  origin.     A  hill, 


252  GEOLOGY 

mountain,  or  lake  basin,  has  its  form  determined  by  a  net- 
work of  geological  forces  and  structures  in  endless  combi- 
nation. 

A.  ELEMENTS  OF  LAND  FORM 
Plains 

241.  Marine  plains. — The  term  plain  is  applied  to  a  com- 
paratively even  surface  which  lies  near  the  level  of  the  sea. 
Such  areas  originate  in  several  ways,  but  marine  plains  are 
the  most  important  type  because  they  are  found  more  or 
less  on  the   borders  of  all  continents  and  may  be  quite 
large.     Such  a  plain  is  a  marginal  sea  bottom,  slightly  ele- 
vated above  the  water  level.     One  of  the  best  illustrations 
is  the  Atlantic  coastal  plain  of  the  United  States,  extend- 
ing from   New  Jersey  to   Florida.     It  consists   of  nearly 
horizontal,  often  unconsolidated,  beds  of  gravel,  sand,  and 
clay.     Two  agencies  are  especially  concerned  in  its  forma- 
tion :  first,  deposition  of  land  waste  and  subsequent  up- 
ward oscillation.     It  has  been  channeled  by  streams  in  a 
moderate  way  since  its  uplift. 

242.  Eiver  and  lake  plains. — A  river,  by  its  windings, 
may  at  length  cut  away  the  hills  and  form  a  plain  of  con- 
siderable width,  which  it  also  strews  with  land  waste.    Such 
plains  often  form  inward  extensions  of  coastal  lowlands, 
as  in  the  case  of  the  Mississippi.     Both  are  covered  with 
land  waste,  but  only  the  coastal  plain  has  been  submerged. 
Lake  plains,  like  those  of  marine  origin,  are  formed  by 
sediments   in   standing   water.     We  find  a  multitude  of 
small  lake   plains   or  lake  floors,  where  the  waters  have 
dried  away  or  been  drained  by  the  lowering  of  the  outlet 
passage.     In  case  of  partial  drainage,  a  lake  plain  borders 
the  remnant  bodies  of  water,  as  about  Lake  Erie  or  Lake 
Ontario. 

Plateaus 

243.  Definition. — A  plateau  is  an  elevated  plain  of  some 
extent.     As  the  term  is  used,  there  is  less  uniformity  of 


PHYSIOGRAPHIC  STRUCTURES  253 

topography  than  in  the  case  of  the  plain.  Considerable 
hills  or  mountains  may  break  the  surface,  which  can  only 
be  called  a  plain  when  great  areas  are  considered.  There 
is  no  limit  of  altitude  fixed  by  Nature,  but  1,000  feet  is 
usually  considered  as  a  dividing  limit.  By  this  standard 
the  so-called  Great  Plains  of  Nebraska,  Kansas,  and  Colo- 
rado form  a  plateau.  The  plains  of  the  Mississippi  merge 
into  it. 

244.  Structure  of  the  rocks  forming  a  plateau. — This  de- 
pends upon  the  structure  of  the  plain  by  whose  uplift  a 
plateau  is  commonly  made.     The  rocks  would  therefore 
sometimes  be  nearly  horizontal,  in  the  altitude  of  original 
deposition.     This   is  the  case  with   the  Catskill  or  Alle- 
ghany  and   Cumberland  plateaus,    stretching  from  New 
York  to  Alabama ;    with  the  great   plateau  east   of  the 
Eocky  Mountains  and  with  the  plateaus  of  the  Colorado 
Eiver.     But  sometimes  the  rock  beds  stand  at  high  angles 
with  the  plateau  surface,  as  in  western  Massachusetts  and 
Connecticut,  in  the  Highlands  of  Scotland  and  the  Ehine. 
In  these  cases  high  mountains  have  been  worn  away,  leav- 
ing a  peneplain,  by  whose  uplift  the  plateau  is  formed. 

245.  Relation  of  plateaus  to  plains  and  mountains. — The 
plateau  may  grade  down  into  the  plain,  as  west  of  the  Mis- 
sissippi Eiver,  or  it  may  descend  by  an  escarpment.     The 
term  escarpment  is  loosely  used,  and  may  refer  to  such  pre- 
cipitous fronts  as  bound  the  Catskill  plateau  on  the  east, 
or  the  steep  hill  slopes  by  which  the  same  plateau  descends 
to  the  plains  of  central  New  York.     Plateaus  may  be  but- 
tressed against  high  mountains  on  one  side,  as  the  Bavarian 
plateau  and  the  Alps,  the  Spanish  plateau  and  the  Pyre- 
nees, and  the   trans-Mississippi   plateau    and  the   Eocky 
Mountains.     Some  plateaus  are  high  basins  between  still 
loftier  mountains.    Such  are  the  Great  Basin  and  the  Colo- 
rado plateau  of  the  "West,  the  plateau  of  Bolivia,  and,  most 
stupendous  of  all,  Thibet,  the  plateau  of  central  Asia,  whose 
altitude  is  as  great  as  that  of  many  Eocky  Mountain  sum- 


254:  GEOLOGY 

mits.  On  the  other  hand,  the  Cumberland  plateau  is  as 
high  as  the  adjacent  Appalachian  Mountain  ridges,  while 
the  Scottish  Highlands  are  not  associated  with  any  higher 
elevations. 


FIG.  131.  -Plateau  with  horizonal  strata,  considerably  trenched  by  valleys. 

246.  Dissection  of  plateaus. — In  consequence  of  high  alti- 
tude, stream  work  and  general  valley-making  are  effective 
in  plateau  masses.     The  exception  to  this  is  found  when  a 
plateau  is   hemmed  in   by  mountain  barriers.     For  this 
reason  the  Great  Basin  plateau  can  not  be  channeled  by 
streams  except  along  the  slopes  about  its  margin.     The 
Colorado  plateau,  on  the  other  hand,  has  been  profoundly 
dissected  by  the  great  river  and  its  branches.     All  stages 
of  dissection  are  found.     The  elevations  of  a  maturely  dis- 
sected plateau  are  often  called  mountains.     Thus  the  Cats- 
kill  portion  of  the  great  eastern  plateau  of  the  United 
States  is  called  the  Catskill  Mountains.     It  is  mountainous 
in  height  but  not  in  structure.     In  Scotland,  however,  we 
have  mountain  height  and  mountain  structure  as  well. 

Mountains 

247.  General  considerations.— The  highest  interest  gath- 
ers about  mountains.    Their  structure  and  origin  have  long 
fascinated  students  of  the  earth  ;  their  beauty  and  majesty 
have  filled  the  thoughts  of  poets  and  of  all  lovers  of  Nature, 
and  they  profoundly  influence  climate,  the  distribution  of 
organic  forms  and  the  history  of  man.     More  than  other 
features  of  Nature,  the  mountains  and  the  sea  leave  their 
impress  upon  character   and   habits.     Compare  the  Swiss 
and  the  Dutch,  while  the  Greek,  the  Scandinavian,  and  the 
Scot  has  often  lived  in  the  presence  both  of  the  mountain 
and  the  sea.    The  study  of  mountains  in  these  relations  be- 
longs to  Physical  Geography  and  History.     We  must  here 


PHYSIOGRAPHIC  STRUCTURES  255 

examine  the  origin  and  form  of  mountains  as  determined 
by  geological  forces. 

248.  Definition. — Mountain  is  an  indefinite  and  popular 
rather  than  strictly  scientific  term,  and  loosely  covers  all 
eminences  which  rise  to  a  considerable  height  above  their 
surroundings.     An  elevation  of  a  few  hundred  feet  rising 
from  a  plain  might  be  called  a  mountain,  while  .in  other 
positions  it  would  be  named  a  hill.     The  best  distinction 
is  that  which  confines  the  term  to  heights  due  to  deforma- 
tion of  the  earth's  crust,  accompanied  by  erosion.    Bearing 
in  mind  this  distinction,  we  may  then  follow  the  popular 
use  of  the  term  as  applied  to  volcanic  cones,  or  high  masses 
carved  out  of  undisturbed  fragmental  beds,  like  the  Cats- 
kill  Mountains,  or  some  of  the  "  table  "  mountains  of  the 
West.     In  this  discussion  we  keep  to  the  more  strict  use  of 
the  word. 

249.  Analysis  of  mountain  aggregates. — A  mountain  range 
consists  of  several  parallel  ridges,  of  considerable  length, 
which  have  been  formed  in  one  epoch  of  disturbance,  or  by 
one  prolonged  crushing  movement.    The  ridges  may  be  the 
original  arches  of  upheaval,  or  anticlinal  folds,  but  more 
often  they  are  outcropping  ridges  of  resistant  rock,  remain- 
ing after  long  erosion.     The  ridges  are  often  of  different 
length,  as  might  be  illustrated  by  the  parallel  folds  of  a 
garment.     The  Appalachian  folds,  extending  from  Catskill, 
N.  Y.,  to  Alabama,  form  a  range ;  likewise  the  Sierras  or 
the  Coast  Eange  of  California.     A  ridge  is  more  or  less 
divided  into  peaks,  depending  upon  the  extent  of  erosion 
and  dissection  which  has  taken  place. 

A  mountain  system  consists  of  several  ranges  in  the 
same  region,  more  or  less  parallel  to  each  other,  but  made 
in  different  periods  of  geological  time.  Thus  the  eastern 
United  States  has  several  times  been  the  theater  of  moun- 
tain-building, and  its  whole  assemblage  of  ranges  is  called 
the  Appalachian  system.  The  student  should  notice  the 
wider  use  of  the  term  Appalachian  in  this  connection.  It 


256  GEOLOGY 

is  unfortunate  that  the  same  word  should  be  used  for  a 
system  and  for  one  of  the  ranges  which  compose  it,  but  the 
usage  is  general.  The  Green  Mountains,  the  Adirondacks, 
and  the  Blue  Eidge  Mountains  are  other  ranges  of  this 
great  system.  Some  authors  use  the  word  chain  for  an 
aggregate  of 'ranges  and  employ  the  word  system  in  another 
sense. 

250.  Origin  of  mountain  ranges. — It  is  not  possible  in  the 
present  state  of  knowledge  to  make  a  complete  statement, 
much  less  in  an  elementary  text-book.  But  students  have 
no  doubt  of  the  general  truth  of  the  origin  of  mountains 
through  the  shrinking  of  the  earth's  crust  and  the  wrink- 
ling of  certain  parts  of  it.  We  must  see  what  sorts  of  rocks 
compose  mountains,  what  forms  these  rocks  have  taken,  and 
how  mountains  lie  with  reference  to  sea  and  land.  It  is 
found  that  thick  masses  of  water-laid  rocks  enter  into  most 
mountain  chains.  The  thickness  is  estimated  at  40,000  feet 
for  the  Appalachians  and  50,000  for  the  Alps.  Many  of 
these  beds  are  sandstones  and  conglomerates,  and  all  are 
rocks  formed  alongshore  or  at  moderate  depths  in  ancient 
seas.  Many  great  mountain  chains,  like  the  Andes,  are 
close  to  the  sea  border,  and  others,  like  the  Appalachians, 
were  close  to  a  sea  border  at  the  time  when  they  were  made. 
Often  a  core  of  crystalline  rocks,  such  as  granite,  is  laid 
bare  by  erosion  along  the  crest  of  a  mountain  ridge,  as  if 
squeezed  up  in  the  time  of  folding.  The  Eocky  Mountains 
furnish  an  example.  The  sedimentary  rocks  of  mountains 
are  often  crystalline  through  metamorphism,  but  in  other 
cases  they  are  unchanged.  This  points  to  a  slow  rate  of 
elevation  and  compression.  Indeed,  if  mountains  were 
growing  in  a  region  it  might  not  be  evident,  save  by  slips 
producing  earthquakes,  or  by  refined  observations  reaching 
over  a  period  of  years. 

Following  Le  Conte,  the  origin  of  a  mountain  range 
may  be  explained  as  follows  :  It  is  found,  as  above  stated, 
that  very  thick  blankets  of  stratified  rocks  compose  the 


PHYSIOGRAPHIC  STRUCTURES  957 

bulk  of  many  mountains.  These  rocks  thin  out  toward  the 
plains.  Thus,  as  we  pass  from  the  Appalachians  to  the 
Mississippi  Eiver  the  pile  of  sediment  is  but  one  tenth  of 
the  thickness  which  it  has  in  the  East,  and  contains  no 
shore  formation.  These  are  the  conditions  that  we  find 
in  going  from  a  sea  border  oceanward.  A  subsiding  sea 
margin  is  the  only  place  where  a  thick  mass  of  coarse  sedi- 
ment could  be  made.  Hence  we  may  be  well  assured  that 
sea  borders  are  the  theater  of  mountain-making.  For  rea- 
sons which  are  less  clear  than  the  fact  itself,  here  seem  to 
be  found  the  weaker  and  more  yielding  belts  of  the  earth's 
crust.  Hence  here  the  strata  are  mashed  together,  and 
there  is  no  relief  save  by  up-and-down  folding  and  crush- 
ing, with  faulting  and  occasional  overthrust. 

251.  Age  of  mountain  ranges. — This  subject  will  be  bet- 
ter understood  as  we   come  to  the  growth  of  particular 
ranges  in  the  course  of  the  earth's  history.     It  is  enough 
here  to  notice  the  general  means  of  fixing  the  period  of 
formation.     It  is  plain  that  if  beds  of  rock  are  folded  and 
broken,  they  were  deposited  before  the  mountain-making 
took  place.     These  broken  and  upturned  beds  may  be  cov- 
ered at  the  base  of  the  mountain  by  beds  which  have  suf- 
fered no  disturbance.     The  mountain,  then,  is  certainly 
older  than  these.     In  other  words,  the  existence  of  an  un- 
conformity helps  us   to  know  within    narrower  or  wider 
limits  when  a   mountain   range    grew.     The   coal-bearing 
rocks  of  eastern  Pennsylvania  are  folded  in,  and  hence  the 
mountains  are  later  than  the  time  of  coal  formation.     But 
the  Mesozoic  beds  are  not  affected,  hence  the  Appalachian 
folds  are  put  between   Paleozoic  and  Mesozoic  time,  and 
their  making  was  the   event  that  in  America   separated 
these  great  divisions  of  geologic  time. 

252.  Form  of  mountains. — A  few  general  principles  only 
need  here  be  stated.     Mountain  form  depends  on  at  least 
four  factors :  (1)  Erosion  forms  peculiar  to  certain  kinds 
of  rock.     Here  Ave  may  cite  the  dome  form  of  many  gra- 


PHYSIOGRAPHIC  STRUCTURES  259 

nitic  mountains,  and  the  rugged  and  often  needle-shaped 
crags  of  the  dolomite  mountains  of  the  Tyrol.  (2)  Bela- 
tive  resistance  of  rock  masses.  We  may  compare  the  Me- 
dina sandstone  ridges  of  Pennsylvania  with  the  lower  slopes 
and  valley  bottoms  of  the  shale  and  limestone,  or  the  crys- 
talline ridges  of  the  Berkshires  with  the  inclosed  limestone 
valley  of  the  Housatonic.  (3)  Attitude  assumed  in  defor- 
mation. Referring  again  to  the  Appalachians,  we  compare 
the  series  of  folds  with  resulting  ridges  and  inclosed  val- 
leys with  the  single  broad  arch  of  the  Uintah  Mountains  in 
Utah  and  Wyoming,  or  we  may  note  the  unequal  slopes  of 
two  sides  of  many  mountain  ridges.  One  side  is  the  escarp- 
ment formed  by  the  broken  edges  of  the  strata,  and  the 
other  is  the  dip-slope  of  the  rock.  (4)  Erosion  as  deter- 
mined by  climate  and  lapse  of  time.  Compare  the  sharp 
summits  of  the  youthful  Alps  with  the  rounded  crests  of 
the  ancient  Adirondacks.  Precipitous  young  mountains, 
with  vigorous  drainage  in  their  valleys,  display  a  rocky, 
rugged  surface.  In  older  mountains  the  inequalities  are 
more  and  more  worn  away ;  the  waste  slopes  stretch  far  up 
toward  their  summits  until  at  last  they  become  perfectly 
"  graded." 

The  declivity  of  mountain  slopes  is  on  the  average 
much  less  than  is  commonly  supposed.  Slopes  appear  to 
the  eye  much  greater  than  they  are,  and  the  attention  is 
fixed  upon  the  more  rugged  parts,  and  withdrawn  from  the 
long  inclines  by  which  the  principal  approach  to  summits 
is  made.  The  Italian  slope  of  the  Alps  is  but  10  feet  in 
100,  and  the  French  side  of  the  Jura  shows  but  2.6  feet 
in  100. 

253.  Typical  examples  of  mountain  rangea— These  will 
illustrate  different  kinds  of  deformation,  particularly  of 
folding,  and  different  degrees  of  destruction  by  erosion. 
We  take  first  the  basin  range  of  the  West,  lying  chiefly  in 
Oregon,  Utah,  and  Nevada.  The  dislocation  is  by  faulting, 
and  the  range  may  be  taken  as  the  type  of  fault  block 


260  GEOLOGY 

mountains.  Their  direction  is  north  and  south,  and  the 
bolder  or  steeper  side  of  each  ridge  is  the  fault  cliff,  while 
the  more  gentle  dip-slope  of  the  rocks  leads  off  from  the 
summit  ridge  on  the  other  side.  The  form  may  be  roughly 
illustrated  by  the  tilting  of  long  ice  blocks  which  are 
crushed  against  each  other,  as  the  entire  mass  breaks  up 
and  moves  with  the  flood. 


FIG.  133.— Section  across  Uintah  Mountains,  showing  a  single  great  fold,  a  fault, 
and  denudation. 

When  mountains  are  due  to  folding,  we  usually  find  a 
number  of  folds  forming  a  range.  The  Uintah  Mountains, 
however,  offer  a  striking  case  of  a  mountain  mass  com- 
posed of  a  single  mammoth  arch.  It  is  cut  through  from 
north  to  south  by  the  caflon  of  the  Green  River,  which  ex- 
poses the  ancient  crystalline  rocks  that  form  the  founda- 
tion of  the  mountain.  But  some  of  the  strata  of  the  great 
blanket  of  later  rocks  are  carried  over  the  top.  The  upper- 
most beds,  to  a  thickness  of  3£  miles,  according  to  Powell, 
have  been  swept  away.  On  dhe  side  the  great  arch  broke, 
and  formed  a  fault  with  a  throw  of  20,000  feet. 

A  variation  from  this  form  is  found  in  the  park  region 
of  Colorado.  The  heights  and  principal  breadths  of  these 
massive  ridges  are  composed  of  crystalline  rocks.  The 
strata  of  the  plateau  on  the  east  and  those  of  the  parks  lie 
nearly  horizontal,  but  their  edges  are  everywhere  bent  up 
against  the  foot  of  the  mountains,  being  ragged  through 
prolonged  erosion.  In  going  from  Colorado  Springs  to 
Manitou,  one  passes  across  the  edges  of  these  broken  beds, 
seeing  them  well  displayed  in  the  shafts  of  the  Garden  of 
the  Gods.  Rising  from  Manitou,  we  at  once  begin  to  trav- 
erse the  crystalline  core  of  the  vast  ridge  which  culmi- 
nates in  Pike's  Peak. 


PHYSIOGRAPHIC  STRUCTURES  261 

In  the  northern  Appalachian  type  we  have  a  range  con- 
sisting of  many  parallel  and  somewhat  open  folds.  Xo 
single  fold  continues  throughout  the  entire  belt  of  disturb- 
ance, though  some  single  anticlines  and  synclines  are  up- 
ward of  100  miles  long.  The  important  feature  here  is  the 
stupendous  erosion  which  has  taken  place,  for  the  moun- 
tains are  old.  Some  of  the  anticlinal  arches  would,  it  is 
believed,  be  from  3  to  5  miles  high  if  destruction  had  not 
gone  along  with  the  uplift  and  until  the  present.  The 
existing  mountain  ridges,  which  inclose  many  long,  canoe- 
shaped  valleys,  only  rise  from  2,000  to  3,000  feet  above  sea 
level.  The  deepest  valleys  are  along  the  axes  of  the  anti- 
clinal folds. 

The  Jura  Mountains  afford  another  illustration  of  open 
folds.  But  they  are  much  younger ;  erosion  and  denuda- 


FIG.  134.— Section  of  the  Jura  Mountains,  showing  ridges  following  the  anticlines 
and  valleys  along  the  synclines. 

tion  are  in  a  comparatively  early  stage,  and  the  ridges  are 
formed  by  the  anticlines  and  the  valleys  run  along  the 
synclines.  Some  of  these  folds  are  remarkably  symmetri- 
cal— that  is,  the  rocks  dip  at  equal  angles  on  either  side  of 
an  axis  of  folding. 

The  Alps  are  a  most  important  illustration  of  mountain 
folds,  and  they  have  been  much  studied.  Here  the  fold- 
ing and  crushing  were  carried  to  a  high  degree,  close  folds, 
overthrown  arches,  faulting  and  stupendous  erosion,  being 
the  rule.  Some  of  the  highest  summits,  as  Mont  Blanc 
and  the  Jungfrau,  are  formed  of  older  crystalline  rocks 
overlying  younger  sedimentary  strata.  This  is  due  to  the 
complete  overthrow  of  gigantic  folds,  Avith  the  inversion  of 
the  beds  which  form  one  limb  of  the  fold.  Several  distinct 
loups  rising  above  one  another  in  zigzags,  can  be  seen  on 
some  of  the  great  cliffs  which  border  the  valleys.  The  in- 
18 


GEOLOGY 


tense  lateral  push  has  so  squeezed  together  the  folds  of  the 
central  Alps  that  these  folds  have  fallen  over  in  either 
direction,  producing  fan  structure  (shown  in  Fig.  135). 
The  Alps  are  young  mountains,  the  valleys  narrow,  the 
slopes  steep,  and  the  peaks  sharp.  Nevertheless,  immense 


FIG.  135.— Section  of  a  part  of  the  Alps,  showing  close  folding  and  fan  structure. 
After  RENEVIBB. 

erosion  has  everywhere  been  accomplished,  and  the  passes 
are  cut  to  about  half  of  the  altitude  of  the  higher  summits. 
In  glacial  times  all  Alpine  valleys  were  occupied  and  much 
modified  by  ice  streams. 

It  remains  to  refer  to  some  very  old  mountains  which  are 
much  worn  and  low.  The  Adirondacks  are  an  example. 
They  are  indeed  higher  than  the  Appalachian  folds,  al- 
though much  older.  This  is  due  to  the  harder  rocks  which 
form  them.  The  New  Jersey  Highlands  and  the  Blue 
Ridge  of  the  Southern  States  are  of  the  same  order,  as  well 
as  the  Highlands  of  Scotland  and  Scandinavia. 

Topography  of  Volcanic  Formations 

253.  A  number  of  the  more  important  facts  were  given 
in  the  last  chapter,  and  in  the  chapter  on  Volcanoes  in 
Part  I.  Thus  we  have  cones  which  are  often  of  mountain- 
ous altitude,  laccolithic  mountains,  volcanic  necks,  and  lava 
sheets.  It  remains  to  emphasize  the  constant  modification 
pf  these  by  destructive  agencies.  In  California  and  in  cen- 


FIG.  136.— Outline  of  various  cones.  1,  Fusiyama.  2,  Hverfjall  (Iceland).  3,  Brac- 
ciano  (crater  lake).  4,  Rocca-Monflna  (Italy).  5,  Teneriffe.  6,  Vulcano,  Lipari 
Islands,  overlapping  cones  along  a  fissure.— After  JUDD. 


264  GEOLOGY 

tral  France  cones  of  some  extinct  volcanoes  are  still  perfect 
in  form.  Others,  in  eruption  in  late  geological  time,  even 
since  the  Glacial  period,  show  considerable  channeling  of 
their  slopes.  Of  these,  Mount  Shasta,  in  northern  Cali- 
fornia, is  a  good  example.  It  is  a  large  cone,  having  an 
altitude  of  14,350  feet  and  a  diameter  at  its  base  of  17 
miles.  Its  slopes  have  an  average  inclination  of  15°,  being 
greater  toward  the  summit  and  much  less  about  the  base. 
We  may  well  compare  Mauna  Loa,  with  similar  height,  a 
diameter  of  70  miles,  and  average  slopes  of  5°.  The  breadth 
of  this  cone  is  due  to  its  very  liquid  lavas.  The  upper 
part  of  Mount  Shasta  still  bears  glaciers  of  considerable 
size.  These  and  their  former  extensions  have  eroded  spa- 
cious cirques,  while  below  is  a  belt  riven  with  great  cafions, 
cut  by  torrents  from  the  glaciers.  Farther  down,  the  drain- 
age runs  beneath  the  surface,  and  a  plane  zone  leads  down 
to  the  general  level. 

A  much  more  advanced  stage  of  erosion  is  described  by 
Dana,  as  shown  by  the  volcanic  island  of  Tahiti,  where  pro- 
found gorges  radiate  toward  the  sea,  bounded  by  precipi- 
tous cliffs,  at  whose  crest  but  a  knife  edge  of  the  general 
slope  remains. 

Following  still  further  the  process  of  destruction,  we 
may  cite  again  Mount  Taylor,  in  New  Mexico,  and  the 
scores  of  volcanic  plugs  in  its  neighborhood. 

Thus  we  find  all  stages  of  topographic  development 
illustrated  by  extinct  volcanic  mountains.  In  a  similar 
way  a  lava  plateau,  like  any  other,  is  at  length  dissected, 
and  nearly  or  quite  destroyed,  the  later  stage  being  shown 
by  the  lava  caps  of  table  mountains. 

Hills  of  Various  Origin 

254.  Hill  is,  even  less  than  mountain,  a  scientific  term, 
but  is  so  often  applied  to  local  elevations  that  it  seems 
best  to  refer  to  them  here.  There  is  obviously  no  fixed 
standard  of  height  as  between  a  mountain  and  a  hill.  As 


266  GEOLOGY 

we  have  seen,  a  worn-out  mountain  country  is  hilly  in  form, 
but  mountainous  in  structure.  A  hill  may  consist  of  a 
pyramid  of  horizontal  beds  left  by  surrounding  erosion. 
Such  isolated  masses  may  be  found  in  the  Alleghany  pla- 
teau of  New  York,  though  most  of  the  plateau  is  made  up 
of  long  hill  ranges  with  alternating  valleys.  Masses  of  vol- 
canic or  Plutonic  rock,  which  resist  erosion  but  are  worn 
to  moderate  altitudes,  form  many  hills.  The  dunes  fur- 
nish another  class,  due  to  the  action  of  the  winds.  All 
these  have  been  sufficiently  described  for  our  present  pur- 
pose. 

255.  Glacial  hills. — These  must  here  be  briefly  described. 
Like  dunes,  they  are  hills  of  accumulation  rather  than 
survivals  of  erosion.  We  here  refer  to  the  glacial  forms 
which  were  left  by  the  great  ice  invasion,  which  are  in 
much  greater  variety  than  those  of  the  Swiss  or  other  val- 
ley glaciers.  The  typical  moraine  is  an  irregular  aggregate 
of  rough  and  often  bowldery  hills,  frequently  forming  a  belt 
of  some  width.  The  slopes  may  be  steep  or  gentle,  and  the 
height  may  vary  from  a  few  feet  to  more  than  1 ,000  in  ex- 
ceptional cases.  A  promiscuous  group  of  steep,  rounded 
knolls,  or  short,  interlocking  ridges  and  knolls,  built  of 
gravel  and  sand,  with  beds  often  highly  inclined,  receives 
the  name  of  kames.  The  inclination  of  the  beds  is  not 
due  to  uplift,  but  either  marks  the  plane  of  deposition,  or 
a  subsidence  due  to  the  removal  of  supporting  foundations 
of  retaining  walls  of  ice,  by  melting.  Kames  are  them- 
selves morainic  structures,  in  whose  formation  the  waters 
of  the  melting  glacier  take  a  large  part.  A  linear  or  ser- 
pentine ridge,  formed  of  material  similarly  stratified,  is 
called  an  esker.  These  occur  in  New  England  and  some 
other  glaciated  regions.  Their  height  varies  much,  even  in 
a  single  example,  often  ranging  from  25  to  100  feet.  The 
side  slopes  commonly  have  angles  of  25°  to  30°,  and  the 
crests  may  be  barely  wide  enough  for  a  road,  for  which 
they  are  not  infrequently  used.  They  follow  the  direction 


268 


GEOLOGY 


of  the  former  ice  movement,  and  were  probably  made  by 
heavily  loaded  streams  flowing  in  subglacial  tunnels,  with 
formation  of  the  side  slopes  as  the  melting  of  the  ice 
allowed  the  materials  to  spread. 

The  drumlin  is  an  elliptical  hill,  whose  longitudinal 
profile  is  a  smooth  curve,  and  whose  cross  section  may 
show  a  rounded  or  somewhat  sharpened  crest.  Drumlins 
occur  in  eastern  Massachusetts,  western  New  York,  south- 
ern Wisconsin,  and  other  places,  and  are  masses  of  till  or 


FIG.  139.— Drumlins  with  marsh  lands,  near  Sun  Prairie,  Wic. 
Contour  interval  20  feet. 

bowlder  clay,  compressed  and  molded  by  overriding  ice. 
Their  longer  axes  coincide  with  the  direction  of  ice  move- 
ment. 

The  movement  of  a  glacial  sheet  across  and  over  hills 
previously  formed  subaerially,  tends  to  reduce  inequalities, 


270  GEOLOGY 

to  plane  off  ridges,  and  to  fill  up  depressions,  thus  giving  a 
smoothed  and  "  linear "  aspect  to  the  topography.  The 
sides  of  hills  may  present  a  fluted  appearance,  and  single 
elongated  hill  masses  are  often  given  a  form  much  like  that 
of  a  drumlin.  The  drumlin  is  a  hill  of  accumulation,  while 
the  drumlinoid  (having  the  form  of  a  drumlin)  hill  is  a 
rock  structure  superficially  modified  by  ice  movement. 

The  Form  of  Valleys 

We  have  already  studied  the  way  in  which  a  valley  of 
erosion  is  made.  The  relation  of  valleys  to  the  general 
elevation  of  a  land  surface  will  receive  notice  in  a  later 
section.  We  consider  here  the  form,  and  first — 

256.  The  cross  section,  or  profile  of  a  valley.— The  chief 
elements  in  this  are  depth,  width,  and  character  of  side 
slopes.  The  depth  depends  on  the  altitude  of  the  land, 
the  time  for  erosive  work,  the  hardness  of  the  beds,  and 
the  vigor  of  the  erosive  agent — river,  glacier,  or  both.  The 
Colorado  Canon  is  deep  because  the  country  is  high,  the 
stream  is  full,  swift,  and  has  been  a  long  time  at  work.  A 
deep  valley  could  not  be  cut  in  the  Atlantic  coastal  plain, 
because  base  level  would  soon  be  reached.  The  width  of  a 
valley  depends  likewise  on  its  age,  the  power  of  the  stream, 
and  the  destructibility  of  the  rocks.  A  valley  may  widen 
indefinitely,  even  in  a  low  country,  if  time  enough  is  given. 

The  character  of  the  side  walls  depends  on  a  great 
variety  of  factors.  They  may  be  nearly  or  quite  vertical 
because  of  overlying  hard  beds,  as  in  the  gorges  of  Kiagara 
or  Trenton,  or  by  reason  of  dominant  joint  planes,  as  in 
Ausable  Chasm.  Eapid  downcutting  with  slight  general 
erosion,  as  in  the  case  of  the  Grand  Cafion  of  the  Colorado, 
tends  to  this  result,  but  it  must  be  remembered  that  only 
the  inner  gorge  is  there  bordered  by  giant  precipices,  while 
above  the  cafion  is  flaring,  often  several  miles  wide.  Ver- 
tical walls  may  bound  a  narrow  or  a  wide  valley  in  the  case 
of  overlying  hard  beds  and  retreat  by  undercutting  of  the 


PHYSIOGRAPHIC  STRUCTURES  271 

softer  beds.  As  we  climb  the  sides  of  some  young  valleys, 
we  may  come  upon  vertical  sections  alternating  with 
benches  or  talus  slopes  due  to  a  succession  of  hard  and 
soft  beds.  A  narrower  inner  valley  may  be  cut  beneath 
the  bottom  of  an  upper  wide  valley.  The  gorge  of  the 
Ehine  is  an  example.  Such  a  case  is  commonly  due  to  a 
relatively  sudden  uplift  of  the  land  by  which  the  stream 
grows  in  velocity,  and  applies  its  energy  mainly  to  down- 
cutting.  Spreading  slopes  of  25°  or  less  are  common 
as  valleys  approach  maturity.  Such  slopes  are  abun- 
dant in  the  valleys  of  the  Alleghany  plateau.  When  a 
valley  has  passed  maturity  its  inclinations  are  much  more 
gentle  and  the  alluvial  deposits  merge  imperceptibly  into 
the  lower  slopes.  Whether  the  two  sides  in  a  valley  profile 
are  symmetrical  or  not  depends  largely  on  the  structure  of 
the  rocks.  A  stream  flowing  along  the  strike  of  a  mono- 
clinal  series  of  rocks  would  have  one  valley  side  steeper 
than  the  other.  The  same  is  true  on  opposite  sides  of  a 
strong  meander. 

Deposits  of  waste,  such  as  the  talus,  terraces,  and  vari- 
ous forms  of  glacial  accumulations,  greatly  affect  the  cross 
profile  of  valleys. 

257.  Longitudinal  profile  of  valleys. — The  vertical  ele- 
ment here  is  so  small  as  compared  with  the  length  of  a 
valley  that  it  is  not  easy  to  form  a  mental  picture  of  it, 
and  it  is  impossible  to  delineate  it  by  a  diagram  without 
gross  exaggeration  of  the  vertical  scale.  The  Colorado,  a 
very  swift  river,  descends  a  little  over  a  mile  in  flowing 
from  Green  River  City  to  the  Gulf  of  California.  The 
Ohio-Mississippi  River  descends  but  600  feet  in  covering 
an  equal  distance  from  Pittsburg  to  its  mouth.  In  general 
the  pitch  of  a  valley  bottom  is  large,  either  by  cascades  or 
torrents,  in  its  beginnings  among  mountains,  moderate  in 
the  long  middle  stretches,  and  almost  zero  toward  the  sea. 
Immature  streams,  which  have  not  smoothed  out  the  in- 
equalities of  their  bed,  may  traverse  almost  level  or  moder- 


272 


GEOLOGY 


ately  descending  platforms,  with  sudden  plunges  of  cataract 
or  rapid  between  them.  In  other  words,  we  have  a  series 
of  local  and  temporary  base  levels.  Local  blockades,  espe- 
cially by  glacial  accumulation,  may  turn  a  stream  from  its 
course  at  certain  points,  and  thus  render  parts  of  a  valley 
immature.  .  We  shall  then  have  the  profile  of  levels  and 
plunges  as  above  described.  A  typical  case  is  the  valley  of 
the  Genesee  Eiver.  A  rapid  head-water  section  near  the 
high  sources  in  northern  Pennsylvania  is  followed  by  a 
mature,  moderately  descending  valley  to  Portage,  N.  Y. 
Then  we  have  a  gorge  and  three  falls,  with  intermediate 


FIG.  142.— Lake  basins  due  to  faulting,  Oregon.     W.  L.,  Warner  Lake ;  A.  L.,  Albert 
Lake  ;  Ch.  V.,  Chemaukan  Valley. 

moderate  descent,  sluggish  flow  in  an  open  valley  to  Koch- 
ester,  two  waterfalls  and  a  gorge  to  Lake  Ontario.  The 
Mississippi,  or  the  Missouri  -  Mississippi  Valley,  gives  us 
head-water  cataracts,  long  courses  of  moderate  descent,  and 
hundreds  of  miles  of  slight  inclination.  In  general,  the 
profile  is  a  curve  or  a  broken  line,  tending  to  become 
straight  with  maturity. 

Lake  Basins 

258.  The  geological  work  of  lakes  was  outlined  in  Part  I. 
The  origin  of  the  basin  involves  so  many  forces  and  struc- 
tures that  the  subject  was  more  conveniently  reserved 
until  now.  We  shall  by  no  means  name  all  the  ways  in 
which  basins  are  formed,  but  for  the  most  part  only  the 
principal  methods.  Fuller  discussion  may  be  found  in 
Russell's  Lakes  of  North  America,  or  in  Davis's  Paper  on 
the  Classification  of  Lake  Basins.*  Lakes  are  signs  of 

*  Proc.  Boston  Soe.  Nat.  Hist.,  1882. 


2Y4  GEOLOGY 

topographic  immaturity.  They  are  transient  features, 
commonly  obliterated  by  filling  with  sediments  or  the  cut- 
ting down  of  outlet  channels.  A  lake  may  disappear  by 
increasing  dryness  of  climate,  but  in  this  case  the  basin 
would  remain. 

259.  Basins  due  to  movements  of  the  earth's  crust. — The 
upthrow  or  downthrow  of  crust  blocks  in  faulting  forms 


FIG.  144. — I,  engineer's  profile  of  Seneca  Lake,  longitudinal.  II,  longitudinal  pro- 
file of  Seneca  Lake  basin  with  vertical  still  much  exaggerated.  Ill,  engi- 
neer's cross  profile.  IV,  actual  cross  profile. 

some  basins  between  adjacent  blocks.  Here  belong  several 
lakes  of  the  Great  Basin  and  the  Dead  Sea  (Fig.  142). 
Downthrow  has  made  the  latter,  and  evaporation  keeps  the 
water  surface  far  below  sea  level,  in  spite  of  copious  supplies 
from  the  Jordan.  Other  basins  lie  between  folds  and  are 
formed  by  them.  A  broad  fold  is  believed  in  part  to  account 


FIG.  145. — Section  of  kettle-hole  ponds  in  a  region  of  kames. 


for  Lake  Superior.  Here  should  probably  be  reckoned  some 
Alpine  lakes,  though  there  is  no  agreement  as  to  the  rela- 
tive efficiency  of  this  cause  and  of  glacial  erosion.  Valleys 
of  erosion  may  be  locally  elevated  or  depressed  by  warping 
of  the  crust,  thus  turning  parts  of  them  into  lake  basins. 
Careful  study  is  needed  for  each  particular  example,  and 
definite  knowledge  may  even  then  be  hard  to  attain. 


PHYSIOGRAPHIC  STRUCTURES 


275 


260.  Basins  due  to  glacial  action. — These  are  of  several 
sorts  and  very  numerous.  Many  thousand  exist  in  the 
United  States.  Some  -are  due  to  blockades  made  by  mo- 
raines left  in  valleys  by  the  retreat  of  the  glacier.  The 
basin  is  thus  a  composite  product  of  river  erosion  and 
glacial  obstruction.  Valleys  may  be  filled  with  ground 
moraine  throughout  part  of  their  course.  To  a  filling  of 
this  sort  south  of  Lake  Ontario  the  Finger  Lakes  of  western 
New  York  are  in  part  due.  Many  small  lakes  lie  in  "  ket- 


Fio.  146.— Glacial  ponds  south  of  Plymouth,  Mass. 
Nearly  20  ponds  in  an  area  of  7  square  miles. 

tie-holes."  These  are  found  among  moraines,  and  are 
believed  to  be  due  in  some  cases  to  a  large  block  of  ice 
which  remained  covered  by  debris  for  some  time,  but  after- 
ward melted  out,  with  subsidence  of  the  cover  and  the 
making  of  a  basin.  Other  similar  basins  are  simply  due  to 
the  irregular  deposition  of  moraines. 

Most  students  of  the  subject  believe  that  many  basins 
have  been  dug  out  of  the  solid  rock  by  glaciers,  forming 
what  is  known  as  rock  basins.  It  is  not  always  easy  to  show 
the  presence  of  a  complete  rock  rim,  or  to  distinguish  a 
glacial  basin  from  one  made  by  folding.  But  it  is  hardly 


276 


GEOLOGY 


reasonable  to  doubt  that  many  basins  of  the  Adirondacks, 
of  Scotland,  and  of  the  Alps  have  this  origin.  The  deeper 
parts  of  the  Finger  Lake  basins  were  almost  certainly  made 
in  this  manner.  Very  careful  search  for  possible  blockade 
should  be  made  before  a  lake  is  affirmed  to  be  in  a  rock  basin. 
261.  Lake  basins  of  various  origin.— It  will  be  sufficient 
to  name  several  of  the  minor  modes  of  formation.  Lakes 


FIG.  147. — Lake  in  glacial  rock  basin.  Scotland. 

are  not  infrequently  due  to  volcanic  action.  Valleys  are 
blockaded  by  lava,  or  water  occupies  an  extinct  crater  or  a 
subsidence  basin,  as  is  supposed  for  Crater  Lake  in  Oregon. 


PHYSIOGRAPHIC  STRUCTURES  27Y 

Many  lakes,  as  on  the  lower  Mississippi,  occupy  old  mean- 
ders, now  cut  off,  or  in  mountain  valleys  are  held  in  by 
debris  cones,  built  by  swift  side  torrents.  Landslips  block- 
ade mountain  valleys  in  a  similar  way.  Inequalities  of  sur- 
face in  great  deltas  produce  shallow  lakes.  These  are  due 
to  the  shifting  of  the  stream  with  its  natural  levees.  Lake 
Pontchartrain  is  a  noteworthy  example.  Small  lakes  are 
due  to  solution  and  sinking  in  limestone  regions,  and  may 
even  occupy  shallow  basins  carved  by  the  wind. 

Shore  Lines 

The  principal  facts  belonging  to  an  elementary  study 
of  these  forms  have  been  given  in  the  chapter  on  the 
Ocean,  and  in  the  account  of  oscillations  of  the  land.  It 
remains  here  to  restate  two  or  three  general  principles. 

262.  A  rising  shore  line. — This  is  commonly  bordered  by 
a  strip  of  coastal  lowland  which,  in  times  geologically  re- 
cent, formed  the  marginal  bottom  of  the  sea.     Inscribed  on 
the  gentle  slopes  of  this  lowland,  or  more  likely  on  the 
base  of  the  older  highlands  behind  it,  platforms  and  cliffs 
may  be  found  which  are  old  shore  lines.     The  present  shore 
is  usually  comparatively  straight,  or  formed  on  a  smooth 
curve,  while  the  adjacent  waters  are  shallow,  and  offshore 
sand  reefs  or  low  islands  are   made  by  wind  and  wave. 
After  long  rising  a  slight  subsidence  may  have  set  in  with- 
out much  changing  the  conditions  which  have  just  been 
described.     Such  is  the  case  with  the  present  coast  of  New 
Jersey. 

263.  A  sinking  shore  line. — Where  sinking  has  long  been 
in  progress  the  water  runs  up  into  the  land  valleys,  and 
the  hills  and  mountains  carved  by  erosive  forces  stand  out 
into  the  sea  as  bold  promontories,  or  form  islands,  washed 
and  trimmed  by  the  waves.     Such  a  shore  line  is  rough  and 
jagged,  the  adjacent  land  topography  has  considerable  re- 
lief, and  the  neighboring  waters  are  often  quite  deep.     Bold 
cliffs  are  formed  by  shore  waves,  abundant  gravel  beaches 

19 


JM 


M  P. 


.  148.— An  uplifted  shore  line  with  coastal  plain.    Davis  and  Curtis  model. 


PHYSIOGRAPHIC  STRUCTURES  279 

are  made  in  protected  places,  and  bars  and  beaches  of  de- 
posit are  carried  across  bays  and  inlets.  Behind  these  bars 
the  waters  fill  with  sediment  from  the  land  and  with  plant 
remains,  and  thus,  by  cutting  away  the  projecting  head- 
lands and  filling  up  the  recesses,  the  shore  line  is  straight- 
ened. This  can  only  happen  if  there  is  a  considerable 
pause  in  the  movement  of  depression.  If  elevation  fol- 
lows long  depression,  the  sea  recedes,  the  shore  lines  form 
marked  features  on  the  seaward  slopes  of  the  land,  and  the 
land  streams  flow  over  and  begin  to  cut  away  the  filling  of 
ancient  bays  and  estuaries. 

264.  Lake  shores. — The  principles  of  shore  formation  are 
the  same  for  lake  and  sea,  save  that  tides  are  practically 
absent  from  the  lake,  and  wave  action  is  relatively  light 
on  small  lakes.  Even  here,  however,  it  is  important.  Lake 
shores  are  remarkably  preserved  on  the  mountain  slopes 
about  Great  Salt  Lake.  This  is  the  remnant  of  a  former 
lake  which  rose  1,000  feet  above  the  present  water  sur- 
face, and  was  two  thirds  as  large  as  Lake  Superior.  By 
reason  of  growing  aridity  of  the  climate,  the  lake,  which 
once  had  an  outlet  toward  the  Snake  Eiver,  has  dried  away 
to  its  present  area,  and  to  its  present  depth  of  about  50 
feet.  The  shore  lines  are  magnificently  preserved  at  sev- 
eral levels  and  are  evident  to  any  traveler  in  that  region. 
The  broad  plains  of  Utah  are  the  bottom  of  this  ancient 
lake.  Those  who  wish  to  know  more  of  this  remarkable 
chapter  in  the  physical  history  of  the  West  should  consult 
Mr.  G.  K.  Gilbert's  report  on  Lake  Bonneville,  published 
by  the  United  States  Geological  Survey. 

Other  conspicuous  beaches  occupy  the  slopes  of  the 
Ked  Eiver  Valley  and  girt  all  the  Great  Lakes  at  various 
levels.  An  account  of  these  will  be  found  in  the  chapter 
on  the  Glacial  period. 


FIG.  149.— A  submerged  shore  line  with  embayed  mountains 
Davis  and  Curtis  model. 


PHYSIOGRAPHIC  STRUCTURES  281 

B.  DEVELOPMENT  OF  A  LAND  SURFACE 

General  Vieio 

265.  We  have  now  reviewed  the   chief  elements  that 
form  parts  of  great  land  areas,  in  the  light  of  their  origin. 
It  remains  for  us  to  see  how  these  combine  and  what 
changes  come  over  the  land  as  a  whole.     Most  land  history 
begins  with  the  spreading  of  the  waste  of  older  lands  on 
the  sea  bottom.     Then  come  uplifts,  slow  in  progress,  turn- 
ing sea  bottoms  at  once  into  low  plains,  or  folding  them 
into  mountain  ranges.     Volcanic  sheets  are  poured  out  here 
and  there  within  or  upon  the  rock  strata,  and  cones  are 
built  high  above  the  surface.     All  the  destructive  forces 
explained  in  Part  I  meantime  attack  the  upraised  masses, 
and  carve  forms  which  depend  in  part  on  the  force  at  work, 
and  in  part  upon  the  structures,  small  and  great,  described 
in  Part  II.     Meantime,  plains  and  mountains,  hills  and 
valleys,  lakes  and  shore  lines,  will  be  seen  everywhere  in  all 
kinds  of  form  and  combination.     But  the  goal  to  which  all 
things  work  is  the  destruction  of  the  lands  and  the  transfer 
of  their  materials  to  the  sea. 

Base  Level  and  Cycles  of  Erosion 

266.  A  base  level  is  a  plane  to  which  denudation  must 
reduce  a  stably  poised  land  mass,  and  below  which  denuda- 
tion can  not  take  place.     This  plane  is  that  of  the  ocean 
surface.     Eocks   are   ever  being   destroyed,   and    gravity, 
frosts,  and  streams  are  moving  their  waste  downward  and 
toward  the  ocean.     The  great  river  first  cuts  its  bed  close 
to  the  sea  level,  and  we  say  that  a  portion  of  the  valley  is 
reduced  to  base  level.     It  lacks  a  little  of  it,  but  the  differ- 
ence is  so  small  that  we  neglect  it.     Gradually  the  valley 
widens,  and  the  base-leveled  strip  extends  up  the  stream 
toward  the  heart  of  the  country.     The  same  process  be- 
gins with  the  lower  and  greater  branches,  while  spurs  of 


282  GEOLOGY 

highland  lie  between  them.  At  length  these  ridges  begin 
to  be  gashed  by  smaller  and  younger  branches,  which  are 
to  the  river  much  as  the  twigs  are  to  the  trunk  and  main 
limbs.  Each  prominent  ridge  of  land  is  buttressed  by 
smaller  ridges  which  run  out  from  it  between  the  streams 
that  head  near  its  crest.  In  time  the  head  waters  interlock, 
the  crests  sharpen  and  their  materials  begin  to  crumble, 
and  the  general  level  of  the  country  begins  to  come  down. 
After  a  time  the  inter-stream  crests  become  rounded  and 
softened,  the  valley  bottoms  wider,  the  relief  of  the  district 
becomes  moderate,  then  faint,  and  base  level  is  approached. 
If  the  country  be  broad  and  high,  inconceivably  long  peri- 
ods of  time  will  be  used,  but  we  must  accustom  the  imagi- 
nation as  much  as  possible  to  the  demand  for  vast  duration 
in  studying  the  history  of  our  planet. 

267.  Rate  of  down  wear  in  earlier  and  later  stages. — While 
plateaus  are  elevated  or  mountains  are  young  and  lofty,  the 
progress  of  denudation  is  rapid.     Let  one  cross  the  plains 
of  Holland  and  think  of  them  as  transferred,  bit  by  bit, 
from  the  Alps ;  let  him  see  the  stupendous  work  of  tor- 
rent and  glacier  in  the  valleys  of  Switzerland,  and  he  must 
then  appreciate  the  swift   destruction  of  elevated  lands. 
As  relief  grows  less,  the  agents  of  destruction  and  carriage 
become  less  active,  and  when  base  level  is  nearly  reached 
the  progress  becomes  very  slow. 

268.  Youth,  maturity,  and  old  age. — These  terms  scarcely 
need  explanation,  but  are  conveniently  applied  to  land  sur- 
faces, showing  various  progress  in  down  wear.     High,  sharp- 
crested  mountains,  with  deep,  narrow  gorges  and  swift  tor- 
rents, belong  to  topographic  youth.     Moderate  altitudes, 
with  a  well-developed  system  of  valleys,  indicate  maturity. 
Low  reliefs,  worn  mountains,  sluggish   streams,  and  the 
absence  of  cliffs,  waterfalls,  rapids,  and  lakes,  belong  to  the 
old  age  of  the  lands. 

269.  Cycles. — A  cycle  of  erosion,  or  geographic  cycle,  is 
the  period  during  which  a  country  of  considerable  relief 


PHYSIOGRAPHIC  STRUCTURES  283 

is  degraded  to  base  level.  Or  the  term  may  be  applied  to 
the  whole  series  of  topographic  forms  which  appear  and 
disappear  during  the  time.  But  it  is  now  important  for 
the  student  to  raise  the  question  whether  the  land,  as  a 
continent  or  large  island,  ever  remains  without  elevation 
or  subsidence  long  enough  for  a  cycle  to  be  finished.  This 
is  certainly  doubtful,  but  approximations  to  base  level  over 
large  areas  have  been  reached.  Such  a  surface,  nearly 
planed  down  to  base  level,  but  retaining  some  hills  or 
mountains  above  the  general  surface,  is  called  a  peneplain 
(almost  a  plane).  Plateaus  like  that  of  central  New 
York,  western  New  England,  the  Highlands  of  Scotland, 
and  the  uplands  of  the  Rhine  are  believed  to  be  ancient- 
ly made  peneplains,  since  uplifted  and  channeled  by  val- 
leys. Any  upland  with  an  even  sky  line,  such  as  may  be 
seen  along  the  mountain  ridges  of  Pennsylvania,  or  in 
the  regions  above  mentioned,  is  likely  to  have  been  pro- 
duced in  this  manner. 

The  Evolution  of  Drainage 

270.  Perhaps  no  geological  force  or  physiographic  form 
is  capable  of  so  much  variety  as  a  river  and  its  valley. 
Many  illustrations  of  this  fact  were  given  in  the  chapter 
on  Rivers.  We  there  studied  the  forms  of  land  waste  in 
valleys,  waterfalls,  revival  by  uplift,  "drowning,"  by  de- 
pression, and  such  accidents  as  glacial  or  volcanic  block- 
ade. In  the  preceding  sections  of  the  present  chapter  an 
account  has  been  given  of  the  form  of  valleys  and  of  the 
more  simple  growth  of  a  river  system  during  the  progress 
of  denudation. 

We  must  now  state  some  further  principles  which  show 
more  fully  the  relation  of  rivers  to  the  lands  and  to  each 
other,  and  which  especially  present  the  river  as  a  historical 
growth.  This  fact  is  emphasized  in  adopting  the  heading, 
Evolution  of  Drainage.  The  title  Adjustment  of  Rivers 
is  sometimes  used.  This  refers  to  the  important  fact  that 


284: 


GEOLOGY 


streams  and  valleys  take  form  and  mutual  arrangement 
from  the  rock  masses  on  which  they  flow. 

271.  Consequent   streams. — We  have  seen  that  the  sim- 
plest and  earliest  kind  of  a  land  surface  is  a  sea  bottom 
which  by  elevation  becomes  a  coastal  plain.     Down  such  a 
gentle  incline  new  streams  begin  to  flow.     They  follow  the 
slope  of  the  newly  made  land,  and  hence  are  called  conse- 
quent streams.     If,  in  the  uplift,  folding  also  takes  place, 
the  streams  will  flow  one  way  or  the  other,  along  the  inclin- 
ing axis  of  the  several  folds.     These  also  are  consequent 
streams— that  is,  they  take  this  course  from  the  original 
form  given  to  the  uplifted  land  surface.     The  streams  of 
the  Atlantic  coastal  plain  illustrate  the  former  case.    Simi- 
lar to  these  are  the  streams  of  the  Eed  River  Valley  in 
Minnesota  and  Dakota.     The  broad  plains  of  this  valley 
are  a  young  lake  bottom. 

272.  Subsequent    streams.  —  Naturally    the    consequent 
streams  do  not  occupy  or  drain  the  entire  surface.     As 
they  establish  themselves,  blocks  of  country  are  left  be- 
tween them  with  imperfect  drainage.    Along  belts  of  weak 
rocks  branches  develop,  which  sometimes  come  to  great 
importance  and  revolutionize  the  river  system  of  the  re- 
gion.    These  new  rivers  are  naturally  termed  subsequent 
streams ;  they  follow,  or  are   secondary  to,  the   original 
drainage.     In  very  ancient  times  the  streams  of  the  south- 
ern Adirondack  slopes  continued  southward  across  south- 
ern New  York,  and  discharged  into  a  sea  that  occupied 
parts  of  Pennsylvania,  Ohio,  and  Virginia.     West  and  East 
Canada   Creeks,   the   Chenango,  and   upper  Susquehanna 
are  modern  representatives  of  these  ancient  rivers.     From 
Albany  westward  by  Utica  extends  a  belt  of  soft,  shaly 
rocks.     It  was  very  easy  for  a  branch  of  the  Hudson  to 
form  and  for  its  head  waters  to  gnaw  backward  in  strata  so 
soft  that  a  winter's  frost  will  reduce  fragments  of  them  to 
their  original  mud.     The  Mohawk  River  is  therefore  a  sub- 
sequent stream,  and  its  trench  is  a  subsequent  valley. 


PHYSIOGRAPHIC  STEUCTUEES  285 

273.  Antecedent  streams. — If  the  whole  area  occupied  by 
a  river  is  uplifted  in  a  somewhat  uniform  way,  the  stream 
is  simply  revived  and  set  to  work  again  in  a  vigorous  man- 
ner.    If,  however,  a  deformation  is  carried  across  its  path, 
it  may  or  may  not  continue  in  its  old  course.    If  the  uplift  is 
too  swift  for  the  down-cutting,  the  stream  will  be  broken  in 
two.     Darwin  found  an  old  stream  bed  in  South  America, 
dry  at  the  time  of  his  visit,  going  down  in  opposite  ways 
from  a  given  point.     But  the  uplift  may  be,  and  usually  is, 
slow,  and  the  stream,  if  powerful,  may  saw  the  growing 
mountain  in  twain  as  fast  as  it  rises  against  it.     The  pas- 
sage of  the  Green  River  through  the  heart  of  the  Uintah 
Mountain  ridge  is  usually  given  as  an  illustration,  but  this 
is  questioned  by  some.     The  Kanawha  River  of  the  Appa- 
lachian region  is  another  example. 

274.  Longitudinal  and  transverse  streams.— Some  streams 
in  a  mountain-built  region  flow  in  the  direction  of  the  folds 
and  others  flow  across  them.     We  will  begin  with  an  exam- 
ple— the  rivers  of  central  and  eastern  Pennsylvania.     The 
West  Branch  of  the  Susquehanna  flows  with  the  line  of 
folding  from  Lock  Haven  past  Williamsport  and  is  then 
transverse  as  far  as  Sunbury.     The  East  Branch  is  longi- 
tudinal most  of  the  way  from  Wilkesbarre  to  Sunbury. 
The  branches  unite  there  and  flow  southward  as  a  trans- 
verse stream,  through  the  great  gap  north  of  Harrisburg. 
Likewise  the  Schuylkill  passes  a  gap  below  Pottsville,  the 
Lehigh  below  Mauch  Chunk,  and  the  Delaware  has  its 
famous  Water  Gap  near  Stroudsburg.     The  upper  waters 
of  these  streams,  and  of  the  Juniata  as  well,  often  flow  for 
long  distances  in  broad,  open  valleys,  northeast  or  south- 
west, and  then  turn  suddenly  at  right  angles  and  pass  the 
mountains  by  narrow  gorges.     Such  an  arrangement  has 
been  called  a  trellised  system  of  drainage.     It  is  thought 
to  have  had  its  origin  when  the  region  was  a  peneplain  near 
sea  level.     The  streams  then  disregarded  the  hard  and  soft 
masses  beneath  them,  but  as  the  land  rose,  being  gently 


286  GEOLOGY 

inclined  southward,  the  wide  longitudinal  valleys  have  been 
etched  out  of  the  soft  rocks",  and  the  vigorous  transverse 
streams  have  kept  pace  in  cutting  away  the  hard  ribs  en- 
countered as  they  have  sunk  their  channels.  By  a  series 
of  changes,  too  elaborate  to  be  explained  here,  longitudinal 
valleys  often  follow  the  eroded  anticlinal  belts.  The  Khone 
is  a  longitudinal  stream  for  most  of  its  course  above  Mar- 
tigny,  and  transverse  from  that  point  to  Lake  Geneva. 

275.  Migration  of  divides.— Territory  may  be  gradually 
or  suddenly  won  from  one  stream  system  to  another.    Such 
changes  have  occurred  in  great  numbers,  and  a  large  field 
of  study  remains  here  for  alert  students. 

If  two  streams  head  against  each  other,  the  upper  waters 
of  one  may  cut  and  carry  away  materials  faster  than  those 
of  the  other.  A  greater  supply  of  rain,  softer  rocks,  or  a 
steep  slope  and  short  distance  to  the  sea,  will  produce  this 
result.  Materials  will  slide  or  be  swept  from  the  crest  over 
into  the  basin  of  the  effective  stream,  and  the  divide  be 
pushed  in  the  opposite  direction.  This  may  go  on  until  the 
vigorous  stream  attaches  to  itself  head-water  tributaries  of 
the  other.  These  tributaries,  instead  of  joining  the  new 
trunk  at  a  natural  angle,  may  resemble  the  barbs  of  a  hook, 
and  their  former  relation  be  detected  in  this  way.  The 
Mohawk  is  now  acquiring  territory  at  the  expense  of  the 
Susquehanna  in  central  New  York.  The  whole  or  a  part  of 
a  stream  running  along  the  strike  of  inclined  beds  will 
migrate  in  the  direction  of  the  dip.  It  should  not  be  in- 
ferred from  the  above  account  that  the  crest  of  a  divide 
must  be  sharp,  with  mountainous  slopes,  in  order  that  effect- 
ive migration  may  take  place. 

C.   CONTINENTS,  OCEAN  BASINS,  AND  THE  GLOBE 

276.  Continents. — Each  great  body  of  land  bearing  this 
name  is  a  combination  of  plains,  plateaus,  mountains,  and 
other  elevations,  the  entire  surface  being  more  or  less  chan- 
neled by  valleys  and  overspread  by  a  network  of  streams. 


PHYSIOGRAPHIC  STRUCTURES  287 

The  height  and  area  of  the  lands  is  small  as  compared 
with  the  depth  and  extent  of  the  seas.  But  few  moun- 
tains are  above  20,000  feet  in  height,  while  important 
groups  like  those  of  the  western  United  States  and  the 
Alps  do  not  vary  much  from  15,000  feet  in  highest  al- 
titudes. Various  estimates  have  been  given  for  the  aver- 
age height  of  continents,  supposing  mountains  to  be  lev- 
eled down  and  plains  to  be  graded  up.  The  estimates 
quoted  by  Dana  are,  in  feet :  Xorth  America,  2,000 ;  South 
America,  1,750;  Europe,  975;  Asia,  2,880;  Africa,  prob- 
ably about  2,000.  The  bulk  of  the  greatest  mountain 
ranges  is  small  as  compared  with  the  general  mass  of  con- 
tinents. 

The  true  bulk  of  the  continental  mass  is,  however,  to 
be  reckoned  not  from  sea  level,  but  from  the  sea  bottom. 
The  coastal  plain  often  descends  very  gradually  under  the 
sea,  and  quite  commonly  there  is  a  somewhat  sharp  descent 
from  the  100-fathom  line.  This  submerged  platform  is 
called  the  continental  shelf,  and  is  50  to  100  miles  wide 
along  much  of  the  border  of  eastern  Xorth  America.  A 
similar  shelf  surrounds  the  British  Islands.  They  are  thus 
structurally  a  part  of  the  European  continent,  with  which 
they  once  had  land  connection.  The  shelf  lying  on  our 
eastern  border  was  also  a  part  of  the  Atlantic  coastal  plain 
in  not  distant  geological  times,  and  the  Hudson,  Delaware, 
Susquehanna,  and  other  streams  were  some  scores  of  miles 
longer  at  their  seaward  ends.  The  submerged  valley  of 
the  Hudson  is  still  traced  by  soundings  out  from  Sandy 
Hook. 

When  we  take  up  the  thread  of  geological  history  we 
shall  see  that  the  great  land  masses  have  long  held  their 
present  positions.  Continents  are  therefore  relatively  per- 
manent, and  have  grown  through  successive  eras  of  de- 
posit and  uplift.  Science  has  no  sure  or  simple  word  to 
say  as  to  the  origin  of  continents,  or  at  least  as  regards  the 
uplift.  Folding  by  contraction  accounts  well  for  the  ele- 


288  GEOLOGY 

vation  of  narrow  mountain  belts,  but  not  clearly  for  lift- 
ing and  sustaining  such  low  and  broad  masses  as  form 
continents. 

277.  Ocean  basins. — The  oceans  are  to  be  regarded  as 
parts  of  the  primeval  waters,  somewhat  isolated  from  each 
other  by  the  formation  of  continents.     Vast  areas  of  the 
central  seas  are  from  2,000  to  3,000  fathoms  deep.     Within 
these  are  smaller  but  important  tracts  having  a  depth  of 
3,000  to  4,000  fathoms.     A  few  patches  are  known  whose 
depth  exceeds  the  last  figure.     Other  great  areas,  especially 
in  high  latitudes  north  and  south,  range  from  100  to  2,000 
fathoms  deep.     The  continental  shelves  and  some  arctic 
seas  fall  within  100  fathoms. 

Widespread  accumulation  is  the  law  of  sea  bottoms,  and 
they  therefore  have  little  of  the  sharp  relief  characteristic 
of  land  surfaces.  The  exceptions  to  this  are  the  volcanic 
and  coral  islands  whose  submerged  slopes  are  of  great 
height  and  often  steep.  Such  a  series  of  islands  as  the 
long  Hawaiian  group,  with  their  submerged  foundations, 
form  an  important  range  of  mountains,  rising  from  the 
profound  depths  of  the  Pacific.  Following  the  axis  of 
the  central  Atlantic  in  a  zigzag  course  is  a  relatively 
broad  ridge,  lying  6,000  to  12,000  feet  below  the  surface, 
bordered  by  the  deepest  parts  of  this  division  of  marine 
waters. 

278.  Form  of  the  earth.— This  has  long  been  known  to 
be  a  sphere  flattened  at  the  poles.     The  amount  of  flat- 
tening is  about  13  miles  at  each  pole.     It  has  been  well 
observed  that  deformation  of  any  circle  in  this  proportion 
could  not  be  detected  by  the  eye.    The  form  is  such  as 
would  be  assumed  by  a  molten  body  cooling  during  con- 
tinuous revolution.     It  is  also  known  that  the  equator  is 
not  a  circle,  but  is  slightly  elliptical.     As  in  the  case  of 
the  polar  flattening,  so  with  ocean  basins  and  mountain 
heights,  the  variations  from  the  form  of  the  sphere  are 
slight.     Many  models  and  diagrams  are  most  misleading, 


PHYSIOGRAPHIC  STRUCTURES  289 

because  depths,  heights,  and  slopes  of  mountains  and  mar- 
ginal sea  bottoms  are  grossly  exaggerated. 

279.  Condition  of  the  earth's  interior. — Several  subjects 
in  previous  chapters  have  led  naturally  to  this  inquiry. 
Among  these  are  volcanoes  and  earthquakes,  oscillations, 
faulting,  and  folding.  It  has  seemed  best  to  defer  any 
notice  of  theories  to  the  present  point.  The  hypothesis  of 
a  molten  interior  formerly  had  general  acceptance.  This 
was  a  natural  result  of  a  known  increase  of  heat  in  deep 
mines  and  borings,  and  of  the  prevalence  of  eruptions  of 
lava  in  past  and  present  times.  The  word  crust  as  ap- 
plied to  the  outer  part  of  the  globe  is  a  remnant  from 
this  belief.  The  term  survives  as  a  convenient  desig- 
nation for  the  rocks  that  are  open  to  study,  and  carries 
no  opinion  about  the  masses  that  lie  below.  The  theory 
of  a  liquid  interior  has  been  given  up  because  of  facts 
made  known  by  astronomy  and  physics.  Under  the  at- 
traction which  produces  the  tides,  the  earth  behaves  like  a 
solid. 

But,  on  the  other  hand,  surface  parts  of  the  earth  can 
not  be  entirely  rigid,  as  is  shown  by  folding,  oscillation, 
and  other  movements  of  the  crust.  Hence  some  suppose 
that  a  relatively  thin  belt  or  zone  of  molten  matter  lies 
below  the  crust,  but  that  the  large  inside  mass  is  solid.  In 
this  view  the  crust  is  solid  by  cooling,  and  the  nucleus  by 
pressure,  while  the  molten  zone  escapes  hardening  by  either 
process.  The  subject  must  be  left  for  the  present,  and 
perhaps  always,  in  doubt. 

It  should  be  remembered  that  a  body  may  be  intensely 
heated  and  yet  remain  solid  under  great  pressure.  This 
is  probably  the  condition  of  the  greater  part  of  the  earth's 
inner  mass.  Crushing  and  folding  may  even  relieve  pres- 
sure at  certain  points,  thus  causing  melting  and  volcanic 
eruptions.  The  density  of  the  globe  as  a  whole  is  about 
5.5.  This  is  nearly  twice  as  great  as  that  of  the  rocks  of 
the  crust.  Some  have  supposed  that  owing  to  their  weight 


290  GEOLOGY 

there  has  been  a  great  concentration  of  the  heavier  metals 
toward  the  center.  Another  view  is  that  ordinary  mate- 
rials are  so  compressed  by  overlying  matter  as  to  have  great 
specific  gravity.  For  more  light  on  this  difficult  question 
geology  must  continue  to  look  to  physics. 


PAET   III 
HISTORICAL    GEOLOGY 


CHAPTEE  XV 

GENERAL   PRINCIPLES 

THE  history  of  the  earth  includes  the  geographical  de- 
velopment of  its  surface  and  the  story  of  its  life.  This 
twofold  theme  we  shall  now  pursue  as  we  might  study  the 
political,  religious,  literary,  or  industrial  history  of  man. 
In  geological  as  in  human  annals  progress  runs  on  differ- 
ent but  parallel  lines.  In  our  field  we  find  a  physiographic 
and  an  organic  evolution.  Incidentally  it  will  be  profit- 
able for  us  to  study  the  chief  economic  products  of  the 
earth  as  we  come  to  some  of  the  periods  of  which  they 
were  formed.  Such  are  coal,  building  materials,  rock  salt, 
and  mineral  oil. 

I.  MATEEIALS  OF  GEOLOGICAL  HISTOKY 

280.  As  in  ordinary  history,  so  we  find  here  a  certain 
range  of  material.  In  the  one  field  the  student  gathers 
from  books,  government  records,  newspapers  of  the  time, 
from  pictures  and  every  kind  of  relics,  and  even  from 
tradition.  Likewise  the  geologist  finds  many  sorts  of 
facts,  and  counts  no  record  unimportant.  It  is  quite 
with  reason  that  many  historical  societies  include  within 
the  same  walls  a  library  and  a  museum  of  natural  ob- 

291 


292  GEOLOGY 

jects.  For  much  that  pertains  to  the  story  of  the  earth 
we  must  go  to  the  astronomer  and  the  physicist,  but  our 
chief  reliance  is  upon  rocks  and  the  remains  of  living 
things. 

We  have  already  seen  that  rocks  can  tell  us  much  about 
the  times  when  they  were  made.  We  know  whether  they 
formed  in  deep  water  or  along  the  shore,  by  organic  or 
mechanical  means,  whether  earth  movements  have  taken 
place,  with  their  nature  and  direction,  and  whether  vol- 
canic commotion  has  combined  with  more  quiet  displays  in 
their  origin. 

Even  more  full  and  important  is  the  revelation  which 
we  may  gather  from  organic  remains.  We  may  know 
whether  the  ancient  creatures  lived  on  the  land  or  in  the 
water,  and  if  the  latter,  whether  they  were  denizens  of 
fresh  waters  or  of  the  sea.  By  knowing  the  habits  of  their 
modern  relatives  we  decide  whether  they  lived  in  the  surf 
or  in  the  deeper  seas.  With  some  limitation  also  ancient 
climates  may  thus  be  known.  The  occurrence  of  corals  in 
the  Northern  States  shows  that  currents  of  warm  sea  water 
once  coursed  in  those  regions,  and  the  finding  of  palms  in 
arctic  latitudes  demonstrates  a  yet  more  surprising  revolu- 
tion of  climate. 

281.  Fossils. — According  to  a  broad  definition,  a  fossil  is 
any  organic  form  buried  in  the  earth  by  natural  causes. 
More  commonly  creatures  thus  inclosed  within  historic  or 
recent  times  are  not  included,  but  no  real  distinction  can 
be  made.  Any  trace  or  impression  of  a  living  thing,  such 
as  a  mold  or  a  track,  is  also  a  fossil.  The  science  which 
deals  with  fossils  is  Paleontology  (Science  of  Ancient  Be- 
ings). On  the  one  hand  it  belongs  to  geology,  on  the  other 
it  is  a  part  of  zoology  and  botany.  The  field  geologist 
generally  submits  his  fossil  specimens  to  a  specialist  in 
paleontology,  and  the  figures  and  descriptions  of  organic 
remains  form  an  important  part  of  the  literature  of  ge- 
ology. 


GENERAL   PRINCIPLES  293 

282.  Preservation  of  land  forms. — The  land  organisms 
suffer  many  chances  of  destruction.     The  oxygen  of  the 
atmosphere  is  ever  causing  decay.     But  there  are  some  cir- 
cumstances that  favor  preservation.     Leaves,  trunks,  and 
entire  plants  may  be  covered  from  the  air  and  saved  from 
decomposition  in  swamps,  or  by  burial  in  flood  deposits, 
and  in  great  marine  deltas.     Likewise  the  bodies  or  skele- 
tons of  men  and  the  higher  animals  are  preserved  through 
miring  in  bogs,  drowning  in  the  passage  of  streams,  or  are 
sealed  up  beneath  cave  deposits.     Worms  and  reptiles  have 
left  their  trails  and  footprints  in  the  muds  of  shores,  to  be 
covered  and  preserved  to  future  ages.     Insects  are  found 
entombed  in  the  fine  muds  of  ancient  lakes,  and  sealed  up 
in  amber,  a  vegetable  gum.     Wood  and  even  prehistoric 
human  implements  are  sometimes  found  in  the  till  and 
gravel  deposited  by  the  glacier  or  glacial  waters.     Twigs, 
leaves,  and  shells  are  often  preserved  in  spring  deposits, 
and  several  kinds  of  land  shells  have  been  found  in  fossil 
tree  trunks  in  the  coal  rocks  of  Xova  Scotia. 

283.  Preservation  of  forms  that  live  in  water. — It  is  in 
river  and  lake  deposits,  and  particularly  those  of  the  sea, 
that  the  most  complete  record  of  life  is  found.     Many  ma- 
rine creatures,  indeed,  are  devoured  as  prey,  or  suffer  decay, 
but  in  immense  numbers  they  are  buried  and  preserved, 
especially  such  as  have  hard  parts,  or  skeletons  of  any  kind. 
They  often  live  in  extensive  colonies,  and  successive  gen- 
erations are  buried  under  sheets  of  sediment  as  they  form. 
Shells  of  pelagic  creatures  (those  living  at  the  surface  of 
the  sea)  go  down  and  mingle  with  those  that  always  remain 
on  the  sea  bottom.     Likewise  in  ancient  lake  deposits,  par- 
ticularly in  the  western  United  States,  a  great  variety  of 
skeletons  has  been  found,  often  of  large  size,  of  mammals, 
reptiles.,  and  birds;     Fishes  also,  and  the  beginnings  of  mod- 
ern vegetation,  are  here  found  in  a  high  degree  of  perfection. 

284.  Fossilization. — This  term  is  applied  to  the  changes 
which  commonly  take  place  in  the  composition  or  structure 

20 


294  GEOLOGY 

of  a  plant  or  animal  during  its  period  of  burial  in  the  rocks. 
These  changes  are  in  all  degrees.  More  recent  fossils  may 
have  suffered  no  apparent  modification.  Others,  such  as 
shells  of  some  geological  antiquity,  may  have  lost  their  lus- 
ter without  much  internal  change.  The  original  materials 
may  suffer  all  degrees  of  replacement.  Thus  a  shell  which 
originally  was  made  of  carbonate  of  lime  may  now  consist 
of  silica.  The  one  mineral  has  been  removed,  particle  by 
particle,  by  infiltrating  waters,  and  the  other  has  taken  its 
place.  The  structure  is  often  perfectly  preserved,  proving 
that  the  change  is  slow.  This  is  illustrated  in  some  speci- 
mens of  fossil  wood,  which  reveal  the  original  woody  struc- 
ture, perfectly  under  the  microscope,  but  are  completely 
petrified. 

Not  infrequently  an  organism  is  more  soluble  than  the 
inclosing  rock.  It  thus  disappears,  leaving  a  cavity  or 
mold,  having  its  own  shape.  This  mold  may  fill  with  other 
matter,  producing  a  cast  which  will  have  the  external  form 
of  the  shell  or  organism,  but  none  of  its  internal  struc- 
ture. We  often  find  also  casts  of  the  interior  of  the  form. 
These  are  sometimes  made  by  mud  drifting  in  between 
the  two  valves  of  a  shell.  Such  specimens  may  be  picked 
up  on  any  beach,  and  are  often  found  in  very  ancient 
rocks. 

285.  Kinds  of  animals  and  plants  most  often  preserved. — 
It  is  plain  that  organisms  having  hard  structures  will  be 
most  sure  to  leave  traces  of  their  existence.  Such  are  the 
bony  skeletons  of  the  vertebrates.  The  skeletons  of  an- 
cient reptiles,  birds,  and  fishes  are  preserved  in  great  per- 
fection. So  also  we  find  the  carapace  of  Crustacea,  such  as 
trilobites,  crabs  and  lobsters,  and  the  integuments  and  ap- 
pendages of  insects.  Shells  of  univalve  and  bivalve  mol- 
lusks  are  preserved  in  countless  numbers  in  almost  the 
oldest  fossil-bearing  rocks,  likewise  the  hard  parts  of  sea 
urchins,  starfishes  and  their  primitive  kindred,  the  crinoids, 
also  of  corals  and  coral-like  creatures.  The  shell-making 


GENERAL  PRINCIPLES  295 

protozoa,  particularly  the  foraminifera  and  radiolaria,  are 
found  fossil  in  abundance,  as  also  woods,  leaves,  and 
fruits. 

It  must  not  be  thought  that  the  rocks  bear  no  evidence 
of  the  more  perishable  creatures.  Worms  are  known  to  go 
far  back,  by  their  trails  and  borings,  and  by  the  jaws  of 
some  species.  Even  the  gossamerlike  jellyfish  has  left  its 
impressions  most  perfectly  upon  surfaces  of  fine-grained 
rocks,  such  as  the  lithographic  limestones  of  Bavaria. 

II.  THE  MAKING  OF  A  HISTOKY 

Having  seen  what  kinds  of  facts  are  at  hand,  we  must 
now  seek  to  know  how  the  geologist  uses  them  to  trace  the 
chain  of  events  in  geological  time.  In  the  midst  of  seem- 
ing confusion,  with  many  kinds  of  rocks  lying  everywhere, 
often  disturbed,  fragmentary,  and  hidden  by  soil  and  forest, 
how  can  a  thread  be  traced  through  the  tangle  ?  This  will 
appear  in  the  chapters  that  follow,  but  a  few  principles  had 
best  now  be  fixed  in  mind. 

286.  Younger  rocks  naturally  overlie  the  older. — This  is 
the  principle  of  superposition,  and  can  be  fully  trusted  in 
the  undisturbed,  sedimentary  rocks  of  a  limited  district. 
As  we  have  seen,  in  an  area  of  great  distortion,  inversion 
may  bring  older  sediments  over  those  that  are  younger. 

The  principle  of  superposition  does  not  help  us  to  con- 
nect the  history  of  one  locality  with  that  of  a  distant  region. 
To  a  considerable  degree  rock  strata  can  be  traced  across 
New  York  from  the  Hudson  Eiver  to  Lake  Erie  and  the 
Niagara  Eiver.  But  even  here  there  are  great  differences. 
Much  greater  are  the  differences  between  the  rocks  of  New 
York  and  those  of  the  same  general  age  in  the  Mississippi 
Valley,  or  in  the  Kocky  Mountains.  Many  strata  are  quite 
continuous  across  England,  from  the  Channel  to  the  North 
Sea.  An  English  geologist  can  make  out  a  history  for 
his  own  country,  as  an  American  might  for  New  York  or 
Pennsylvania,  but  on  the  principle  of  superposition  the  two 


296  GEOLOGY 

could  make  no  comparison  of  their  results  with  a  view  to 
tracing  the  history  of  the  globe. 

287.  The  succession  of  living  creatures. — This  is  the  most 
important  fact  in  making  a  history  of  the  earth.  By  much 
study  and  comparison  during  the  past  century,  geologists 
have  found  -that  certain  kinds  of  creatures  lived  in  the  time 
when  very  ancient  rocks  were  made.  Other  kinds,  begin- 
ning to  have  more  resemblance  to  modern  animals  and 
plants,  lived  in  what  we  may  call  the  middle  periods,  and 
successive  groups  have  led  gradually  on  to  the  forms  of  to- 
day. Thus  a  kind  of  standard  series  from  earliest  to  pres- 
ent types  has  been  made  out,  so  that  now,  if  a  new  fossil  is 
found,  it  can  be  referred  to  its  place  in  the  known  series, 
and  this  settles  the  question  of  the  relative  age  of  the  rocks 
that  contained  it. 

Two  or  three  examples  may  be  given.  The  trilobite,  as 
shown  by  wide  observation,  is  practically  confined  to  what 
is  known  as  the  Paleozoic  era.  If  a  specimen  is  found  in 
place,  this  determines  the  rocks  as  of  this  time.  But  it  is 
an  era  of  immense  duration.  We  find,  however,  that  cer- 
tain kinds  of  trilobites  appear  only  in  certain  earlier  or 
later  parts  of  that  era.  Thus  we  narrow  the  age  of  the 
rocks  to  closer  limits,  and  say,  strata  containing  the  tril- 
obite called  Paradoxides  are  Cambrian,  and  if  they  have 
Asaphus,  they  are  Lower  Silurian.  Oysters  are  not  older 
than  Mesozoic,  and  particular  kinds  of  oyster  shells  belong 
to  various  strata  down  to  the  present  time.  Of  course,  the 
standard  scale  is  open  to  revision  by  fresh  discoveries,  as 
when,  a  few  years  ago,  the  present  director  of  the  United 
States  Geological  Survey  found  fish  remains  in  older  rocks 
than  had  before  been  thought  to  contain  them.  But  we 
may  trust  the  correctness  of  the  succession  as  a  whole.  It 
is  no  man's  invention,  but  has  grown  up  out  of  the  re- 
searches of  thousands  of  observers  in  all  parts  of  the  world. 

288.  Progress  not  uniform. — In  the  early  days  of  geology 
it  was  thought  that  the  earth  had  been  shaped  mainly  by 


GENERAL  PRINCIPLES  297 

great  catastrophes.  This  false  idea  gradually  gave  way  to 
the  doctrine  of  continual  progress  in  past  ages  as  at  pres- 
ent. This  is  the  true  view,  but  must  not  be  pushed  too 
far.  As  there  are  epochs  of  revolution  or  of  swift  political 
or  social  unfolding,  so  there  have  been  times  of  relatively 
swift  geographic  change.  Great  changes  in  geography 
cause  also  changes  in  the  living  forms.  Barriers,  such  as 
mountains,  are  reared  where  none  were  before.  Sea  chan- 
nels are  closed  and  others  opened,  the  direction  of  currents 
and  the  temperature  of  the  waters  are  changed,  clear  waters 
are  clouded  with  sediment,  and  variations  of  depth  are 
caused.  All  these  changes  subject  living  forms  to  great 
strain,  and  they  must  migrate,  modify  their  habits,  or 
perish. 

289.  Prophecy  and  reminiscence. — No  movement  of  his- 
tory begins  or  ends  abruptly.     There  were  beginnings  of  a 
Rocky   Mountain  range  long  before  the  chief  upheaval. 
This  is  especially  true  in  the  history  of  life.     Fishes  were 
once  thought  to  begin  and  to  become  abundant  in  the 
Devonian  period.     But,  as  we  have  seen,  their  beginnings 
were  far  earlier.     Reptiles  were  the  sovereigns  of  Mesozoic 
lands  and  seas.     But  there  were  forerunners  of  the  tribe  in 
the  Carboniferous  times,  and  the  reptiles  of  to-day  are  a 
minor  group,  a  reminiscence  of  those  ancient  days.     In 
other  words,  no  age  stands  alone.     It  comes  out  of  the  past ; 
it  leads  into  the  future ;  it  marks  a  step  in  the  long  and 
never-resting  evolution  of  the  world. 

290.  Faunas  and  floras. — The  assemblage  of  animals  liv- 
ing at  a  particular  time  or  age  of  the  world's  history  is 
called  the  fauna  of  that  period.     We  may  also  speak  of  the 
fauna  of  a  particular  group  of  rocks,  or  of  a  limited  region, 
as  the  fauna  of  the  Rocky  Mountains,  of  the  British  Islands, 
of  the  deep  seas.     The  Devonian  fauna  of  New  York  em- 
braces all  the  animals  living  in  this  area  in  the  Devonian 
period,  whose  remains  have  been  preserved  in  the  rocks. 
Or  we  may  confine  the  term  to  particular  groups  of  animals, 


298  GEOLOGY 

such  as  the  fish  fauna,  the  molluscan  fauna,  the  inverte- 
brate fauna.  The  term  flora  is  used  in  precisely  the  same 
manner  of  groups  of  plants. 

291.  The  life  period  of  species  and  of  the  larger  groups. — 
The  great  types  of  animal  life,  except  the  vertebrate,  are 
found  in  strata  which  are  among  the  oldest  sediments  pre- 
served to  us.  The  backboned  animals  began  later,  but 
in  high  antiquity.  But  the  minor  groups  have  had  a 
shorter  history.  They  have  come  and  gone ;  have  thrived 
for  a  time  and  given  way  to  others.  Of  the  vertebrates, 
fishes,  amphibians,  reptiles,  and  mammals  have  one  after 
another  come  into  prominence,  and  all  exist  to-day,  though 
some  are  of  diminished  importance.  But  within  these 
classes  many  subdivisions  have  long  been  extinct.  The 
larger  branches  of  the  organic  tree  usually  live  the  long- 
est— that  is,  classes  and  orders  persist  longer  than  genera 
and  species,  which  are  the  included  smaller  groups,  and  less 
distinct  from  each  other.  As  it  has  taken  long  to  develop 
the  fundamental  types,  so  they  have  great  vitality.  But 
genera  and  species  yield  to  modifying  influences,  and  come 
and  go. 

These  principles  will  be  constantly  used  as  we  proceed, 
but  a  single  illustration  at  this  point  may  be  useful.  The 
Brachiopods  are  a  highly  important  group  throughout 
Paleozoic  time.  Hundreds  of  genera  and  species  populated 
the  ancient  seas.  Many  appeared  in  the  Cambrian  period. 
Some  of  these  (Lingula)  have  persisted  with  little  change 
until  to-day.  Another  group,  the  Spirifers,  did  not  appear 
until  the  Upper  Silurian  period.  They  became  very  abun- 
dant, but  were  nearly  extinct  at  the  end  of  the  Paleozoic 
era.  But  certain  species  of  Spirifers  are  found  only  in  a 
single  stratum  or  small  group  of  strata.  They  were  off- 
shoots of  the  main  stock,  which  for  unknown  reasons  ran 
their  course  and  were  soon  extinct.  It  is  also  an  important 
principle  that  a  species  once  extinct  has  never  been  known 
to  reappear. 


GENERAL  PRINCIPLES  299 

292.  The  geographical  distribution  of  fossil  species.— As 

with  existing  animals  and  plants,  so  it  is  with  those  of 
former  times.  Some  flourish  in  a  narrow  district,  while 
others  extend  over  a  wide  field.  We  often  can  not  under- 
stand this,  for  one  may  appear  to  be  as  vigorous  as  the 
other.  In  the  case  of  wide  distribution,  the  form  is  be- 
lieved to  have  developed  in  a  single  region,  and  thence 
gradually  to  have  spread  over  land  or  sea.  Such  spread- 
ing is  called  migration.  It  is  not  meant  that  individuals 
change  their  home,  except  in  certain  cases,  as  of  birds,  but 
that  successive  generations  widen  the  area  occupied,  or  con- 
tract it  on  one  side  and  extend  it  on  the  other,  as  when  a 
forest  is  said  to  retreat  before  growing  glacial  conditions. 

III.  DIVISIONS  OF  GEOLOGICAL  TIME 

293.  Limits  put  between  epochs  of  history  are  always 
more  or   less  arbitrary.     The  classical  times  merged  into 
the  mediaeval,  and  the  Middle  Ages  led  gradually  up  to  the 
modern  centuries.     Certain  nations  have  been  powerful,  or 
certain  social,  political,  or  religious  conditions  have  pre- 
vailed during  a  period.     Or  some  great  single  event  may  be 
a  suitable  boundary,  as  when  we  use  the  American  Eevolu- 
tion  as  marking  the  end  of  colonial  days  and  the  beginning 
of  federal  government  in  our  own  country. 

In  like  manner  the  earth's  history  is  one,  but  some  spe- 
cial progress  in  land-making,  or  some  type  of  living  things, 
may  distinguish  a  period  of  time.  Thus  the  Devonian 
period  is  sometimes  called  the  Age  of  Fishes.  It  is  not 
meant  that  other  creatures  did  not  live  in  great  abundance, 
but  only  that  fishes  were  then  first  numerous,  and  were  the 
highest  animals  of  the  time.  Similarly  the  Mesozoic  era 
is  often  called  the  Age  of  Eeptiles.  The  great  event  which 
in  America  marks  the  passage  from  Paleozoic  to  Mesozoic 
time  is  the  Appalachian  revolution,  by  which  a  great 
chain  of  mountains  was  made  in  the  east.  Similar  disturb- 
ances occurred  in  the  European  area  at  about  the  same 


300  GEOLOGY 

time.  We  thus  have  a  suitable  landmark  to  separate  two 
great  areas.  But  we  must  also  remember  that,  even  then, 
quiet  prevailed  in  many  parts  of  the  world.  Change  is 
always  going  on,  with  special  events  here  and  there.  For 
the  purpose  of  study  we  analyze  the  progress  as  best  we  can. 

294.  Equivalent    strata    in    regions    remote    from    each 
other. — These  can  only  be  determined  by  comparison  of 
fossils.     These  need  not  be  of  the  same  species,  and  usually 
are  not.     But  closely  resembling   species  and  genera  are 
found — that  is,  the  same  orders,  classes,  and  types.     This 
does  not  show  that  the  beds  were  formed  at  exactly  the 
same  time,  but  approximately,  for  time  must  have  elapsed 
for  the  species  or  genera  developing  in  one  place  to  migrate 
to  the  others.     The  important  principle  for  us  to  remem- 
ber is,  that  beds  with  similar  forms  are  contemporaneous 
in  a  general  way.     Thus  the  larger  divisions  of  time  are 
the  same  for  all  countries,  but  the  smaller  belong  to  a 
single  country  or  district.     The  Upper  Silurian  period,  for 
example,  is  well  represented  by  the  rocks  of  England  and  of 
New  York.     But  in  England  the  subdivisions  of  the  Silu- 
rian are  called  Llandovery,  Wenlock,  and  Ludlow,  while  in 
New  York  we  speak   of   Medina,   Clinton,  Niagara,  and 
Salina.     The  sets  of  subdivisions  are  only  at  some  points 
alike,  as  regards  their  rocks  and  fossils.    It  will  be  seen 
that   such  subordinate  divisions  are  usually  named  from 
places  where  the  rocks  are  well  displayed.     The  student 
should  also  notice  that  we  may  apply  the  name   either 
to  the  rocks,  or  to  the  interval  of  time  during  which  they 
were  made,  and  during  which  their  fossils  were  parts  of 
living  organisms. 

295.  Tabular  view  of  geological  time. — The  names  of  the 
eras  and  periods  as  here  given  are  nearly  all  in  universal 
use.     The  same  is  true  of  the  epochs  of  the  Cenozoic  era. 
The  epochal  names  given  for  Mesozoic  and  Paleozoic  belong 
to  the  American  formations  which  represent  those   eras. 
The  names  of  the  eras  refer  to  successive  stages  in  the  life 


GENERAL   PRINCIPLES  301 

of  the  globe.  Paleozoic  means  ancient  life.  Mesozoic 
refers  to  the  mediaeval  era  of  organic  history,  but  is  in 
reality  far  later  than  this  as  regards  the  passage  of  time. 
The  Cenozoic  is  the  era  of  new  or  modern  life. 

The  names  of  the  periods  have  come  into  general  use 
gradually  and  without  similar  harmony  of  meaning.  For 
example,  Carboniferous  refers  to  a  prominent  mineral  char- 
acter of  the  deposits  of  the  period ;  others,  like  Devonian, 
indicate  a  locality  where  the  formations  are  well  developed 
and  were  first  studied ;  and  some,  as  Tertiary,  are  the  sur- 
viving remnants  of  numerical  subdivisions  which  were 
much  used  in  the  early  days  of  geology.  Each  name  will 
be  explained  in  the  appropriate  connection.  In  some  text- 
books the  term  period  is  used  of  certain  shorter  divisions 
of  the  Paleozoic  era.  Uniformity  in  the  use  of  names  is 
convenient,  but  the  lack  of  it  does  not  affect  the  reality  of 
the  time  intervals  or  of  the  formations  made  while  they 
were  passing. 

TABLE 

CENOZOIC  ERA  \  Quaternary  Period,  or  Pleistocene  Epoch. 

(  Tertiary  Period. 
r  Cretaceous  Period. 

MESOZOIC  ERA -|  Jurassic  Period. 

I  Triassic  Period. 
Permian  Period. 
Carboniferous  Period. 

PALEOZOIC    ERA ^   Devonian  Period. 

Upper  Silurian  Period. 

Lower  Silurian  Period. 
I  Cambrian  Period. 
ARCH.EAITAND  ALGOXKIAN  ERAS. 

The  student  will  find  it  well  to  become  perfectly  familiar 
at  the  outset  with  the  names  and  order  of  the  eras  and  pe- 
riods, beginning  with  the  oldest ;  also  with  epochs  whose 
rocks  may  occur  in  his  own  State  or  region.  For  any  full 
study  of  the  epochs  resort  must  be  had  to  the  larger  text- 
books and  to  geological  reports. 


CHAPTER  XVI 

ARCH^JAN   AND  ALGONKIAN   BRAS 

296.  THE  beginnings  of  the  earth's  history  can  never 
be  known  from  record  or  relic.  But  the  facts  and  com- 
parisons afforded  by  astronomy  save  us  from  complete 
ignorance.  What  we  may  with  considerable  safety  infer 
about  the  earliest  condition  of  the  globe  is  suggested  by 
the  Nebular  Hypothesis.*  This  hypothesis,  first  proposed 
by  Laplace,  supposes  that  all  the  matter  of  the  solar  sys- 
tem was  once  a  revolving  mass  of  gases,  having  a  diameter 
equal  to  that  of  the  outermost  planet's  orbit.  With  the 
condensation  that  took  place  rings  were  thrown  off  like 
those  of  Saturn,  and  these  rings  broke  up  into  planets  with 
their  satellites.  The  present  sun  is  the  central  remnant  of 
the  original  vast  nebula.  Many  facts  support  the  theory. 
Such  are  that  more  than  two  hundred  bodies  of  our  system 
have  the  same  direction  in  their  orbits,  that  their  satellites 
pursue  a  like  direction,  which  is  the  same  taken  by  each 
individual  body  in  rotating  upon  its  axis ;  and  that  the  sun 
is  slowly  contracting  its  diameter.  Nebulas  marking  vari- 
ous stages  of  cooling  and  condensation  are  believed  to  have 
been  found  in  the  celestial  spaces. 

If  events  took  such  a  course,  the  earth  must  have  been 
glowing  and  molten  for  a  long  period  before  an  outer  crust 

*  For  a  criticism  of  the  Nebular  Hypothesis,  and  a  cogent  and  inter- 
esting presentation  of  an  alternative  meteoroidal  theory,  see  article  by 
T.  C.  Chamberlin,  Hypotheses  bearing  on  Climatic  Changes. — Journal 
of  Geology,  October-November,  1897. 
302 


ARCHAEAN  AND  ALGONKIAN  ERAS  393 

could  be  formed.  But  this  condition  must  come  at  length, 
for  the  seething  and  fiery  mass  was  constantly  losing  its 
heat  into  cold  spaces.  At  last  it  would,  at  least  in  places, 
cease  to  glow,  and  lavalike  crusts  would  form,  only  to  be 
broken  up  from  time  to  time  and  be  reabsorbed,  enacting 
on  a  gigantic  scale  what  now  transpires  in  the  lava  pools  of 
Hawaiian  craters.  Even  after  a  tolerably  continuous  crust 
had  formed,  volcanic  outbursts  must  have  been  frequent 
and  stupendous.  Until  cooling  had  well  progressed,  the 
gases  which  now  make  up  the  waters  of  the  globe  were  in- 
cluded in  the  atmosphere,  which  would  have  been  dense, 
dark,  and  of  great  thickness.  It  also  contained  great  meas- 
ures of  carbon  since  stored  in  rocks,  and  rested  with  heavy 
weight  upon  the  earth.  During  this  stage  no  organic  life 
was  possible. 

Gradually  the  crust  became  more  stable,  the  gases  con- 
densed, and  the  primeval  seas  came  into  being.  These  may 
have  been  at  first  universal,  but  this  we  can  not  know. 
Wherever,  by  folding  and  upheaval,  igneous  rocks  rose 
above  the  waters,  lands  were  formed,  rains  would  fall, 
streams  would  come  into  being,  and  the  age-long  processes 
of  erosion  and  sedimentation  would  begin.  If  the  waters 
of  the  sea  were  not  still  everywhere  hot,  they  must  often 
have  been  locally  heated  by  volcanic  outbursts.  Such 
water,  with  a  moist  and  hot  atmosphere,  would  carry  on 
chemical  changes  with  intensity  now  unknown  on  the 
surface  of  the  earth. 

The  time  of  a  gaseous,  molten,  and  glowing  earth, 
before  there  was  a  solid  crust,  has  been  called  by  Dana 
the  "  Astral  "  aeon  or  era.  By  the  same  author  the  term 
Azoic  (without  life)  is  applied  to  the  time  of  the  first 
crust,  of  high  temperature,  widespread  waters,  first  emerg- 
ing lands,  and  dense  atmosphere.  But  as  all  subdivisions 
must  be  here  vague  and  arbitrary,  we  only  make  such  dis- 
tinction as  is  possible  between  the  earlier  period  which  was 
barren  of  life  and  the  later  time  from  which  a  few  fossils 


304:  GEOLOGY 

have  been  preserved.  If  beginnings  seem  dark  and  our 
knowledge  doubtful,  the  student  must  remember  that  such 
is  the  case  even  where  human  history  merges  into  the  pre- 
historic but  a  few  thousand  years  ago.  From  the  entire 
pre-Christian  era  even  a  less  body  of  fact  is  known  than 
may  be  gathered  from  the  single  century  now  closing. 

297.  Archaean  Era. — This  term,  which  means  ancient  or 
primitive,  is  applied  by  some  authors  to  all  of  pre-Paleozoic 
time  and  its  rock  formations.  Most  American  geologists 
now  restrict  the  word  to  the  time  of  the  earlier  complex 
masses  of  highly  crystalline  rocks,  many  of  them  igneous, 
which  are  the  oldest  rocks  preserved  to  us.  It  is  not 
probable  that  any  of  these  are  parts  of  the  first-formed 
crust.  It  is  hardly  possible  that  any  part  of  that  crust 
could  survive.  The  Archaean  rocks  contain  no  fossils,  but 


c 

FIG.    150. — Diagrammatic   section    showing   unconformity   between   Archaean   and 
Algonkian  (a  and  b)  and  between  Algonkian  and  Paleozoic  (b  and  c  d). 

they  have  suffered  such  prolonged  and  severe  metamorphism 
and  disturbance  that  organisms,  if  ever  present,  would  have 
been  destroyed.  Doubtless  many  of  these  rocks  were  origi- 
nally igneous,  but  some  may  have  been  sedimentary.  No 
thickness  can  be  assigned  to  them,  but  they  underlie  later 
rocks  everywhere,  and  form  the  surface  in  irregular  areas 
of  small  or  large  extent,  where  they  have  been  uncovered 
by  denudation. 

297i-  Algonkian  Era. — The  Algonkian  rocks  are  younger 
than  the  Archaean,  and  are  largely  sedimentary.  They  lie 
unconformably  on  or  against  the  Archaean.  This  shows 
that  a  very  ancient  land  surface,  roughened  by  erosion, 
received  a  cover  of  bedded  rocks.  But  this  later  series  is 
greatly  metamorphosed,  disturbed,  and  broken  in  its  turn. 


ARCHAEAN  AND  ALGONKIAN  ERAS  305 

It  gives  a  better  clew  to  its  origin  by  means  of  bedding 
planes,  often  preserved  and  containing,  rarely  indeed,  some 
obscure  fossils.  The  general  distribution  of  pre-Paleozoic 
rocks,  as  a  whole,  is  considered  in  the  following  section. 
Some  of  the  important  formations  distinctly  recognized  as 
Algonkian  are  here  noted.  The  Huronian  rocks,  held  by 
Dana  and  others  as  a  later  division  of  the  Archgean,  are 
included  in  the  Algonkian.  The  beds  first  known  as 
Huronian  occur  to  the  north  of  Lake  Huron,  and  consist 
(Van  Hise)  of  comparatively  little  altered  quartzites,  slates, 
slate-conglomerates,  cherts,  and  limestones,  having  a  thick- 
ness of  18,000  feet.  A  great  series  of  Algonkian  rocks 
occurs  in  the  Lake  Superior  region.  Here  belongs  the 
Keweenawan  formation,  having  a  maximum  thickness  of 
50,000  feet.  Algonkian  rocks  occur  in  many  parts  of  Canada 
and  of  the  western  United  States.  The  subdivisions  of  the 
Algonkian  in  Minnesota  are  given  as  Keewatin,  Animike, 
and  Keweenawan,  in  ascending  order.  Taking  the  pre- 
Paleozoic  rocks  as  a  whole,  they  can  only  be  compared  in 
remote  areas  or  different  continents,  by  saying  that  they  are 
all  alike  older  than  the  earliest  Paleozoic  formations.  These 
latter  contain  many  fossils,  and  can  be  correlated  even 
across  the  seas.  Of  the  duration  of  the  Archaean  and  Al- 
gonkian eras  we  can  only  make  the  general  statement  that 
it  was  immensely  long.  That  the  cooling  of  the  surface 
would  require  vast  time  is  clear  from  the  slow  rate  of  cool- 
ing of  thin  streams  of  lava  flowing  out  upon  a  cold  crust. 
We  must  add  to  this  the  long  development  of  organic  life, 
which  probably  took  place  before  the  Paleozoic  era  began. 

298.  Areas  of  pre-Paleozoic  rocks. — By  this  expression  we 
mean  not  actual  outcrop,  but  regions  whose  bed  rock,  more 
or  less  covered  by  the  soil,  is  of  this  age.  More  extensive 
than  all  others  combined,  in  North  America,  is  the  great 
Canadian  area  extending  from  Labrador  down  upon  the 
Great  Lakes  and  thence  northwestward  to  the  Polar  Sea.  It 
forms  a  great  V,  which  holds  Hudson  Bay  between  its  arms. 


306  GEOLOGY 

In  northern  Michigan,  Wisconsin,  and  the  Adirondacks  are 
its  southern  extensions.  As  seen  above,  a  vast  series  of  the 
later  Algonkian  formations  is  found  in  the  Lake  Superior 
district.  The  mining  interests  of  this  region  have  led  to 
careful  study  of  its  rocks,  but  in  much  of  North  America, 
and  especially  of  the  other  continents,  no  line  has  yet  been 
drawn  between  Archaean  and  Algonkian.  Pre-Paleozoic 
rocks  form  the  most  ancient  axis  of  the  Appalachian  Moun- 
tain system,  being  found  in  the  Highlands  of  New  York 
and  New  Jersey,  and  continuing  southwestward  into  Geor- 
gia. Other  belts  extend  from  New  England  to  Nova  Scotia 
and  Newfoundland.  Considerable  belts  are  found  at  the 
heart  of  the  Rocky  Mountain  range  and  in  other  parts  of 
the  mountain  system  of  western  North  America.  Patches 
appear  in  the  Black  Hills,  in  Missouri,  and  in  central  Texas. 
Everywhere  these  rocks  have  formed  mountains,  which  are 
often  worn  off,  as  in  the  Highlands  of  the  Hudson,  to  low 
altitudes.  They  therefore  made  land  areas  of  considerable 
extent  in  these  very  ancient  times.  But  we  must  not  sup- 
pose that  these  lands,  in  form  or  size,  were  the  same  as  the 
pre-Paleozoic  areas  of  to-day.  Submergence  and  elevation 
doubtless  affected  lands  then  as  now,  and  there  was  ample 
time  for  many  fluctuations  during  the  eras. 

The  important  fact  to  observe  is,  that  the  continent  was 
roughly  sketched  in  by  these  early  lands.  Lowlands  now 
extend  from  the  Gulf  of  Mexico  to  the  Arctic  Ocean,  over  a 
region  which  even  then  was  swept  only  by  shallow  seas.  This 
emphasizes  yet  again  the  very  early  origin  of  our  continent. 

299.  Pre-Paleozoic  life. — Here  direct  proof  is  meager,  but 
several  facts  lead  strongly  to  the  belief  that  living  creatures 
of  lowly  kinds  dwelt  in  these  ancient  seas  for  a  very  long 
period.  But  few  fossils,  and  these  generally  obscure,  are 
found.  This,  however,  is  to  be  expected ;  for  most  of  the 
primitive  forms  were  probably  soft  and  fragile,  and  meta- 
morphism  has  gone  on  so  long  that  it  is  wonderful  to  find 
any  fossils  remaining  in  these  rocks.  Beds  of  crystalline 


ARCHAEAN  AND  ALGONKIAN  ERAS  307 

limestone  are  common,  and  limestones  are  usually  formed 
through  the  agency  of  living  creatures.  Beds  of  iron  ore 
are  extensive,  and  iron  is  usually,  at  least,  concentrated  by 
means  of  organic  products.  Graphite  and  shales,  rich  in 
carbon,  point  to  the  same  conclusion.  According  to  Dana, 
the  plants  were  mainly  algae  and  microscopic  fungi.  Geol- 
ogy has  no  light  to  throw  upon  the  beginnings  of  living  mat- 
ter. It  only  knows  that  organic  history  as  a  whole  points 
to  a  progress  from  lower  to  higher  through  the  long  ages, 
and  this  argues  strongly  for  a  very  long  pre-Paleozoic  era, 
during  which  the  most  lowly  forms  of  plants  and  animals 
were  leading  the  way  toward  the  abundant  life  disclosed  by 
the  Paleozoic  formations.  That  the  previous  records  should 
have  been  nearly  destroyed  is,  as  we  have  seen,  inevitable. 

300.  Economic  products  of  pre-Paleozoic  rocks.— The  iron 
ores  of  northern  and  eastern  New  York  and  of  New  Jersey 
have  long  been  worked.     Likewise  the  iron  of  northern 
Michigan  and  of  Missouri  belongs  to  this  era.     In  these 
rocks  occur  also  tin,  as  in  South  Dakota,  and  Cornwall, 
England;    much   copper,   gold,  platinum,  mica,  graphite, 
and  apatite  (calcium  phosphate,  used  as  a  fertilizer).     The 
Keweenawan  formation  contains  the  rich  copper  deposits 
of  northern  Michigan.     Building  stones  are  found  in  abun- 
dance.    Such  areas  are  often  important  for  their  forests, 
their  supplies  of  water,  and  the  beauty  of  their  scenery. 

301.  Summary. — Cooling,  condensation,  the  formation  of 
seas,  atmosphere  and  early  lands,  are  the  features  of  the 
pre-Paleozoic  eras.     At    some    point,  to  the  geologist  un- 
known, life  had  its  beginnings,  and  may  have  been  abun- 
dant at  the  close  of  the  time.    If  we  had  a  full  record,  these 
eras  could  doubtless  be  separated  into  important  divisions. 
Emphasis  should  be  placed  upon  this  point.    Otherwise  the 
student  may  gain  the  notion  that  these  intervals  were  short, 
because  we  know  so  little  about  them,  and  because  any 
account  must  therefore  be  brief.     What  we  mean  by  long 
duration  in  geology  will  better  appear  as  we  proceed. 


CHAPTER  XVII 
PALEOZOIC  ERA 

CAMBRIAN  PERIOD 

302.  Introduction  to  Paleozoic  history. — We  enter  here 
upon  an  era  whose  records  are  comparatively  full  and  easy 
to  read.  It  is  like  passing  from  the  traditions  and  molder- 
ing  remains  of  our  aboriginal  Indian  tribes  to  the  abundant 
annals  which  we  have  of  our  colonial  days.  Even  here  much 
has  been  lost,  but  in  one  library  or  another  every  essential 
fact  can  probably  be  found.  So  it  is  with  Paleozoic  history. 
Over  the  broken  and  much  modified,  older  rocks,  lie  the 
sandstones,  shales,  and  limestones  of  the  various  Paleozoic 
periods,  often  packed  with  fossil  remains.  Sometimes, 
indeed,  these  rocks  are  also  changed  by  metamorphism, 
and  confused  by  dislocation,  but  we  are  still  upon  historic 
ground,  and  we  may  be  sure  that  future  study  will  only 
confirm  the  principles  which  we  now  hold. 

Paleozoic  time  is  very  long.  It  is  certainly  to  be  reck- 
oned in  millions  of  years,  and  perhaps  by  tens  of  millions. 
In  its  beginning  there  were  islands  of  earlier  formed  lands, 
roughly  tracing  the  continents  that  were  to  be.  At  its 
close  there  were  some  large  continental  areas,  and  the 
interior  or  mediterranean  seas  were  growing  shallow,  as  in 
the  region  west  of  the  Mississippi  River.  All  through 
Paleozoic  time  land  waste  was  accumulating  in  the  seas 
south  of  the  present  Great  Lake  region  and  west  of  the 
Appalachian  Mountain  axis.  By  filling  on  the  sea  borders 
and  by  occasional  upward  oscillations  the  interior  sea  was 


PALEOZOIC  ERA  309 

shrinking  in  breadth  and  depth,  until  the  entire  eastern 
area  of  our  country  was  dry  land.  The  West,  on  the  other 
hand,  remained  a  region  of  sea  and  islands,  a  single  Paleo- 
zoic land  mass  of  importance  developing  where  now  is  the 
Great  Basin.  The  details  of  this  geographic  unfolding  are 
reserved  for  the  following  chapters. 

303.  Paleozoic  life.— In  the  earlier  parts  of  the  era  all 
life  was  in  the  seas,  and  it  was  wholly  invertebrate.     Can 
we  picture  those  early  days  ?    Barren  enough  the  landscape 
must  have  appeared,  had  there  been  a  human  eye  to  see  it. 
There  may  have  been  lands  of  bold  height,  for  mountains 
were  made  in  pre-Paleozoic  time.     But  there  was  no  green 
meadow  or  waving  forest,  no  insect,  flower,  or  bird.     Over 
an  uncarpeted  surface  denudation  went  on  vigorously  and 
muddy  streams  crossed  the  dark  lands  to  the  sea.     In  the 
sea  no  fishes  swam  and  its  monarchs  were  the  trilobite  and 
the  orthoceras.     The  climate  was  warm  and  moist,  the  at- 
mosphere heavy  and  full  of  clouds. 

As  we  pass  the  middle  of  the  era,  land  forms  begin  to 
come  in,  both  animals  and  plants,  but  both  were  still  sub- 
ordinate to  their  kindred  in  the  sea.  Perhaps  before  the 
middle  of  the  time  we  come  upon  the  earliest  traces  of  the 
great  vertebrate  group  which  in  man  was  to  dominate  all, 
but  as  yet  we  have  only  a  prophecy.  When  the  era  closed, 
however,  there  were  wide  lands,  luxuriant  forests,  there 
were  insects  to  awaken  vibrations  of  the  air,  the  amphibian 
had  come  and  a  few  primitive  reptiles,  while  fishes  of  an- 
cient patterns  swarmed  in  the  seas.  But  of  trees  and 
flowers  of  modern  kinds  there  were  none.  It  was  still  a 
Paleozoic  world. 

304.  Relation  of  Paleozoic  to  earlier  rocks. — It  is  almost 
everywhere  the  relation  of   unconformity.     Very  general 
upheavals    have    taken    place,    and    the    waste  was    laid 
down  to  form  the  foundations  of  the  Paleozoic  systems. 
Sometimes  the  decayed  surfaces  of  the  older  mass  are  still 
to  be  found  when  the  newer  beds  are  stripped  away.    They 

21 


310  GEOLOGY 

are  bits  of  ancient  land,  interesting  relics  of  pre-Paleozoic 
geography. 

305.  The  Cambrian  period — origin  of  the  name. — British 
formations  of  this  period  were  especially  studied  by  Sedg- 
wick,  of  the  University  of  Cambridge,  between  1830  and 
1860.     They  occur  in  Wales,  and  hence  were   named  by 
him  Cambrian,  from  the  early  name  of  that  region.     The 
Cambrian  period  has  sometimes  been   counted   as  a  sub- 
division of  the  period  that  follows,  but  fuller  discoveries 
both  in  Europe  and  America  have  justified  its  claim  to 
stand  by  itself. 

306.  Epochs  of  the  Cambrian  period. — In  America  these 
are  as  follows.     The  name  of  the  earlier  epoch  is  placed 
below  to  correspond  with  the  relative  position  of  the  rocks  : 


rt, 

' i 


3.  Potsdam  Epoch. 

CAMBRIAN  PERIOD ]  2.  Acadian  Epoch. 

Georgian  Epoch. 


The  Georgian  epoch  is  named  from  a  typical  series  of 
its  rocks  near  Georgia,  Vt. ;  the  second  epoch  bears  the 
name  of  a  Canadian  locality  near  St.  Johns,  New  Bruns- 
wick ;  and  the  third  is  taken  from  the  town  in  northern 
New  York  about  which  its  formations  are  finely  displayed. 
The  epochs  and  their  rocks  are  also  called  Lower,  Middle, 
and  Upper  Cambrian. 

307.  Cambrian  formations.— The  rocks  of  the  Cambrian 
period  consist  of  shales,  sandstones,  and  conglomerates, 
with  only  occasional  limestones.  That  they  are  commonly 
coarse,  fragmental  beds  shows  that  they  were  laid  down  in 
shallow  waters  of  the  sea  border,  and  accordingly  we  find 
them  chiefly  in  narrow  belts  fringing  the  pre-Paleozoic 
areas  in  the  North,  the  East,  and  the  West.  They  also 
underlie  the  younger  formations  generally,  and  are  some- 
times revealed  in  such  situations  by  profound  erosion,  as  in 
the  Grand  Cation  of  the  Colorado.  As  may  be  inferred 
from  the  above,  the  surface  areas  of  Cambrian  rock  in  North 


PALEOZOIC  ERA  31 1 

America  are  numerous,  but  individually  of  small  extent. 
Only  the  worn  edges  are  for  the  most  part  exposed.  Thence 
they  descend  beneath  the  younger  members  of  the  several 
systems.  That  the  exposed  beds  are  sea-border  formations 


FIG.  151. — North  American  land  areas  in  Cambrian  time.  8  is  shown  as  extended 
eastward.  The  dotted  line  is  conjectural.  The  student  must  not  take  such  a 
map  as  trustworthy  in  details.  Compare  similar  map  in  Dana's  Revised  Text- 
book of  Geology,  p.  237,  on  which  the  Adirondack  and  other  minor  areas  are 
shown.— After  LE  CONTB. 

is  also  shown  by  the  prevalence  of  ripple  marks,  rain  prints, 
mud  cracks,  and  the  trails  of  marine  animals  upon  their 
surfaces. 

Cambrian  rocks  are  found  in  Newfoundland,  Nova 
Scotia,  and  New  Brunswick.  Eocks  of  this  period  occur 
along  the  borders  of  the  pre-Paleozoic  areas  of  New  Eng- 
land and  New  York.  At  Georgia,  Vt.,  is  the  typical  lower 
Cambrian.  This  is  adopted  as  the  type  section,  by  reason 
of  the  fossils,  especially  the  trilobites,  which  are  there 


312  GEOLOGY 

found.  At  Braintree,  Mass.,  are  Middle  Cambrian  slates 
and  conglomerates,  famous  for  their  fossils,  particularly 
the  trilobite  Parodoxides.  At  other  points,  fossiliferous 
beds  of  this  age  are  associated  with  metamorphic  schists 
and  marbles.  The  Potsdam  (Upper  Cambrian)  sandstones 
of  New  York  are  among  the  best  known  of  American 
Cambrian  formations.  They  outcrop  around  the  base  of 
the  Adirondacks,  consisting  of  reddish  shales  and  sand- 
stones, from  60  to  several  hundred  feet  thick.  Near 
Saratoga  Springs  is  a  Cambrian  (Potsdam)  limestone  with 
many  fossils.  Cambrian  strata  continue  along  the  Appa- 
lachian axis  to  Georgia.  In  Pennsylvania,  in  the  South 
Mountain  region,  ancient  lavas  prove  volcanic  activity 
during  the  period.  Other  representatives  are  found  in 
northern  Michigan  and  Wisconsin.  Cambrian  rocks  ap- 
pear in  the  Black  Hills,  in  the  Rocky  Mountains,  are 
about  800  feet  thick  near  the  bottom  of  the  Grand  Caflon, 
and  have  an  important  development  in  the  Great  Basin  in 
Nevada. 

308.  Life  in  the  Cambrian  period. — We  have  seen  that 
the  fossils  of  pre-Cambrian  rocks  are  few  and  obscure,  but 
from  limestones  and  other  deposits  we  infer  that  life  was 
abundant.  But  in  the  Lower  Cambrian  alone  we  find  at 
least  170  species  described  for  North  America.  Before 
the  end  of  the  period  all  the  great  types  were  present, 
except  vertebrates.  We  have  here,  however,  not  an  ab- 
rupt appearance  of  living  things,  but  only  a  more  perfect 
record.  The  general  unconformity  between  pre-Paleozoic 
and  Paleozoic  rocks  shows  widespread  disturbance,  and  a 
general  blurring  of  the  record.  Such  disturbances  of  land 
and  sea  also  would  rapidly  change  the  faunas  of  the  time. 
Some  species  would  die  out,  and  others  would  be  much 
modified. 

Several  classes  of  fossils  now  to  be  named  will  be  de- 
scribed in  the  account  of  the  Lower  Silurian  period,  in  which 


PALEOZOIC  ERA  313 

they  appear  in  great  perfection.  No  sure  representatives 
of  Protozoa  have  been  discovered,  though  they  doubtless 
existed,  but,  being  small  and  fragile,  are  lost.  A  few 
sponges,  graptolites,  and  corals  are  found.  Echinoderms 
are  represented  by  Cystids,  a  less  perfect  relative  of  the 
ancient  Crinoids.  Bivalve  and  univalve  mollusks  are  not 


FIG.  153.— Dicellomus,  a  Cambrian          FIG.  153.— Lingulella  cselata,  dorsal  valve, 
Brachiopod.  enlarged.      Lower  Cambrian. — After 

WALCOTT. 

uncommon,  but  are  few  as  compared  with  their  numbers 
in  the  following  period.  Of  creatures  with  shells,  the 
Brachiopods  are  far  the  most  important.  They  soon  multi- 
ply to  many  hundreds  of  species  and  continue  to  be  a  great 
host  in  the  seas  throughout  the  Paleozoic  era.  A  few  lead- 
ing genera  of  the  Brachiopods  should  be  carefully  remem- 
bered as  the  student  goes  on.  The  most  abundant  Cam- 
brian genus  is  Lingulella,  with  its  related  forms.  The  shell 
is  thin  and  delicate,  similar  in  size  and  shape  to  a  small  fin- 
ger-nail, except  that  it  is  often  pointed  at  the  apex.  While 
most  Brachiopod  shells  are  made  of  carbonate  of  lime,  this 
consists  of  lime  phosphate.  In  some  rocks  it  is  ebony- 
black  in  color,  with  a  shining  surface,  which  is  marked  by 
fine  lines  concentric  with  the  beak  or  apex.  Some  slabs  of 
Cambrian  rock  are  almost  covered  with  these  shells.  They 
have  the  great  additional  interest  of  having  survived  with 
little  change  to  the  present  time,  while  other  Brachiopods, 
much  more  abundant  in  a  given  period  or  place,  have  been 
for  millions  of  years  extinct.  No  good  reason  has  ever 


314 


GEOLOGY 


been  assigned  for  these  vast  differences  in  the  vertical  dis- 
tribution or  life  period  of  different  forms. 

309.  The  most  important  creature  in  the  Cambrian  seas 
was  the  trilobite.  It  belongs  to  the  class  of  Crustacea,  ani- 
mals with  outside  skeletons,  body  rings,  and  jointed  append- 
ages. As  •  the  name  implies,  the  body  has  three  lobes  or 
ridges,  separated  by  two  furrows,  which  are  sometimes  deep 
and  sharp  and  sometimes  obscure.  Each  lobe  is  divided 
into  rings  or  segments  (Fig.  154).  Three  parts  of  the  body 

are  also  distinguished 
— a  head,  the  abdomen, 
and  the  tail.  There 
is  great  variety  as  to 
the  distinctness  of  the 
lobes,  and  rings  in  head 
and  tail.  Sometimes 

—  ^^^  _    the  rings  of  the  abdo- 

jf  /?*  '  111('n    :lll(l    tail,   or   the 

yf    /  |    rear  angles  of  the  head 

'  shield,  are  prolonged 
into  sharp  spines. 
Prominent  stalks  on 
the  head  shield  some- 
times bear  eyes,  con- 
sisting of  many  lenses 
(see  eye  of  Phacops 
rana,  section  338),  but 
in  other  cases  the 
creatures  were  blind. 
In  some  species  the 
segments  moved  freely 
upon  each  other,  and 
the  animal  could  roll 
itself  together,  conceal- 
ing its  under  parts  within.  Many  fossil  specimens  are 
found  in  this  condition.  Adult  trilobites  vary  in  length 


FIG.  154.—  Olenellns  (Mesonacis)  Aeaphoides. 
Lower  Cambrian,  Washington  County,  N.  Y. 
—After  WALCOTT. 


PALEOZOIC  ERA 


315 


from  less  than  an  inch  to  two  feet.  They  are  a  Paleozoic 
group.  We  shall  meet  many  forms,  and  find  them  of  high 
importance  as  marking  strata  of  different  periods  and 
epochs. 

Over  50  species  of  trilobite,  according  to  Walcott,  have 
been  found  in  the  Lower  Cambrian  rocks  of  North  America. 


PIG.  155.—  Protypus  Hitchcocki. 


FIG.  156.— Paradoxides  Harlani. 
J  natural  size. 


Not  only  are  the  Cambrian  trilobites  peculiar  to  the  period, 
but  some  genera  are  characteristic  of  the  several  epochs. 
Paradoxides  Harlani  has  long  been  known  from  the  Middle 
Cambrian  rocks  of  Braintree,  Mass.,  and  specimens  of  it 
may  be  seen  in  the  collection  of  the  Boston  Society  of 
Natural  History  and  in  other  museums. 

Other  crustaceans  occur  in  Cambrian  rocks,  and  tracks, 
sometimes  obscure,  made  by  crustaceans,  mollusks,  or  worms. 
Some  doubtful  impressions  also  are  referred  to  seaweeds, 
which,  without  much  doubt,  existed.  But  we  have  no 
record  of  any  land  plant  or  animal. 

310.  North  American  geography  in  the  Cambrian  period. — 
According  to  Walcott,  there  was  extensive  land  in  the  cen- 


316 


GEOLOGY 


tral  portions  of  the  United  States  in  the  Georgian  or  Lower 
Cambrian  epoch.  Hence  deposits  of  that  age  could  not  be 
formed  in  that  region.  During  the  later  Cambrian,  how- 
ever, sinking  went  on  and  the  sea  crept  northward  over  the 
interior  of  our  continent.  In  harmony  with  this  we  find 
thick  masses  of  coarse,  fragmental  rock  on  the  borders  of 
the  ancient  lands  lying  around  this  central  area,  and  these 


Fia.  157.— Ripple- 


rked  Potsdam  sandstone  with  trilobite  trails,  Port  Henry,  N.  Y. 
(In  New  York  State  Museum.) 


beds  could  only  have  gained  such  thickness  through  conti- 
nental subsidence.  But,  notwithstanding  subsidence,  the 
interior  sea  remained  shallow  in  the  later  Cambrian  times, 
showing  that  virtually  a  great  continental  mass  lay  where 


PALEOZOIC   ERA  3^7 

North  America  now  is.  This  testifies  to  the  great  an- 
tiquity and  permanence  of  our  continent.  That  the  ac- 
tual land  of  Cambrian  time  was  extensive  and  somewhat 
bold  is  shown  both  by  the  abundance  and  the  coarseness 
of  the  marine  sediments  of  the  period. 


FIG.  158.— Trail  of  animal.    Upper  Cambrian  of  Arizona.— After  WALCOTT. 

311.  Economic  products  of  Cambrian  formations. — The  red 

sandstones  of  Lake  Superior,  and  the  red,  chocolate,  and 
cream-colored  sandstones  of  the  Potsdam  epoch  in  New 
York  are  much  used  for  building.  The  latter  are  particu- 
larly durable,  being  compacted  by  a  siliceous  cement,  and 
have  shown  a  maximum  crushing  weight  of  42,000  pounds. 
They  will  also  endure  a  degree  of  heat  which  is  destructive 
to  marbles  and  granite.  Cambrian  rock  is  also  quarried  for 
marble  at  Swanton,  Vt. 


CHAPTEE  XVIII 
PALEOZOIC  ERA 

LOWEK  SILUKIAN  PERIOD* 

312.  Name  and  subdivisions. — The  name  of  the  period  is 
taken  from  an  ancient  British  tribe,  the  Silures,  and  was 
first  used  by  the  English  geologist,  Murchison,  who  studied 
this  system  of  formations.  The  epochs  are  as  follows  : 

!5.  Hudson  Epoch  (Lorraine  beds). 
4.  Utica  Epoch 
3.  Trenton  Epoch. 
2.  Chazy  Epoch. 
1.  Calciferous  Epoch  (Beekmantown  limestone). 

Before  and  after  the  year  1840  several  eminent  geol- 
ogists made  a  survey  of  the  State  of  New  York.  They 
found  a  full  and  generally  undisturbed  succession  of  Paleo- 
zoic formations,  and  usually  applied  to  them  the  names  of 
localities  where  they  were  well  displayed.  Thus  we  have 
what  has  become  known  as  the  New  York  series  of  rocks. 
This  has  been  adopted  as  a  standard  of  comparison  for  all 
the  Paleozoic  formations  of  North  America.  Thus  the 
Trenton  limestone  is  found  typically  exposed  in  the  great 
gorge  at  Trenton  Falls,  N.  Y.  The  time  during  which 
the  rock  was  made  is  the  Trenton  epoch.  If  a  formation 
of  the  same  relative  age  is  found  in  the  West  or  South, 
it  is  said  to  be  of  Trenton  age.  It  may  also  have  a 
local  designation,  and  may  consist  of  shale  or  sandstone 

*  By  some  authors  this  period  is  called  Ordovician. 
318 


PALEOZOIC   ERA  319 

rather  than  limestone.  It  must,  however,  have  the  same 
position  as  the  Trenton  in  the  geological  column,  as  shown 
by  its  fossils  and  by  its  general  relations.  By  attention  to 
this  principle  the  student  may  avoid  the  feeling  of  confu- 
sion which  often  attends  the  occurrence  of  so  many  local 
names  of  formations. 

313.  General  character  of  the  Lower  Silurian  period. — We 
have  seen  that  in  Cambrian  times  the  rocks  made  in  the  re- 
gions now  accessible  to  us  were  mainly  coarse  and  often  of 
shore  formation.     The  student  must  not  forget  that  in  the 
deep  seas  of  that,  as  of  all  periods,  fine  muds  were  accumu- 
lating.    In  the  Lower  Silurian,  however,  the  region  of  the 
growing  continent  was  largely  covered  by  waters  of  some 
depth,   often   clear  and  free  from  land  waste,  in   which 
Brachiopods,  corals,  and  Crinoids  could  flourish,  and  con- 
tribute  their  remains  toward  the  making  of  limestones. 
Hence  the  Lower  Silurian  was  largely  a  limestone-making 
period.     This  does  not  mean  that  the  waters  had  great 
depth,  which  is  not  necessary  for  the  accumulation  of  ordi- 
nary limestones.     Only  moderate  subsidence  was  needed, 
and  this,  as  we  saw,  was  in  progress  in  the  later  Cambrian. 
At  the  close  of  the  period  there  were  mountain-building 
in  the  East,  and  extensive  additions  to  the  growing  con- 
tinent. 

314.  Epochs  of  the  Lower  Silurian  period.— The  earliest  is 
the  Calciferous,  and  its  rocks  overlie  the  Upper  Cambrian. 
In  this  case  the  name  is  not  a  local  one,  but  was  applied  in 
New  York  to  a  sandy  rock  containing  much  lime.     A  typ- 
ical locality  for  the  rocks  and  fossils  of  this  epoch  is  at  Beek- 
mantown,  N.  Y.     The  weathering  away  of  the  lime  is  apt 
to  leave  a  surface  roughened  by  the  outstanding  grains  of 
quartz.    Middle ville  and  other  calciferous  localities  in  New 
York  are  famous  for  the  perfection  of  their  quartz  crystals. 
Equivalents  of  the  Calciferous,  as  shown  by  fossils,  are 
found  in  Newfoundland,  also  southward  to  Tennessee  and 
westward  to  Iowa  and  Minnesota. 


320  GEOLOGY 

The  Chazy  epoch  is  represented  by  limestones  in  the  St. 
Lawrence  region,  and  in  northeastern  New  York,  where  a 
village  furnishes  the  name.  Purer  limestones  following — 
that  is,  overlying — the  Calcif erous  beds,  show  deepening  and 
clearing  of  the  waters.  The  most  important  epoch  of  the 
Lower  Silurian  is  the  Trenton.  It  will  help  the  student 
to  clearer  ideas  of  the  growth  of  the  continent  if  we  state 
more  fully  the  distribution  of  the  Trenton  limestones. 
Over  300  feet  of  them  are  exposed  in  the  gorge  of  the  West 
Canada  Creek  at  Trenton  Falls.  They  are  shaly,  thin- 
bedded,  and  rich  in  fossils,  except  a  heavy  crystalline  stra- 
tum at  the  top.  In  the  vicinity  and  along  the  Black  Eiver 
thin  strata  of  the  dark  Black  Kiver  limestone,  and  the  light- 
gray  Birdseye  (Lowville)  limestone,  form  the  base  of  the 
Trenton.  The  formation  outcrops  along  the  Mohawk  Val- 
ley, and  more  or  less  about  the  base  of  the  Adirondack  mass. 
It  is  well  developed  in  the  St.  Lawrence  region,  and  extends 
up  an  ancient  marine  gulf  to  Ottawa.  A  belt  of  Trenton 
stretches  along  the  north  shore  of  Lake  Ontario.  West- 
ward, it  is  found  in  Wisconsin,  Minnesota,  Iowa,  and  Mis- 
souri. Eeturning  to  New  York,  we  find  it  southward  along 
the  Appalachian  axis.  It  is  brought  to  light  by  extensive 
erosion,  and  forms  the  floor  of  the  great  anticlinal  valleys, 
like  the  Nittany  in  central  Pennsylvania,  and  is  several 
hundred  feet  thick  in  Tennessee.  Borings  for  gas  and  oil 
have  shown  that  the  Trenton  extensively  underlies  the 
younger  formations  in  central  and  western  New  York,  and 
in  large  areas  of  Ohio  and  Indiana.  Thus  we  can  picture 
the  interior  sea,  teeming  with  organic  life  which  covered 
its  bottoms  with  a  mantle  of  calcareous  mud.  The  borders 
of  the  sea  lay  north  and  east,  near  the  present  lines  of  out- 
crop of  Trenton,  Cambrian,  and  pre-Paleozoic.  Coarser 
rocks,  which  must  have  been  made  along  the  actual  shores 
of  the  Trenton  sea,  have  been  destroyed  by  the  denuding 
forces  which  have  been  so  long  at  work.  These  would  have 
carried  the  real  shore  line  in  central  New  York,  for  exam- 


PALEOZOIC  EEA  321 

pie,  farther  to  the  northeast  on  the  Adirondack  slopes. 
Eocks  referred  to  the  Trenton  epoch  occur  in  various  parts 
of  the  Kooky  Mountain  region,  in  the  Great  Basin,  and  in 
arctic  latitudes. 

In  Xew  York  especially  the  waters  began  to  be  clouded 
with  fine  materials  which  settled  over  the  Trenton  muds 
and  formed  a  thin-bedded  black  shale  from  100  to  700  feet 
thick.  The  time  during  which  this  deposit  was  in  progress 
is  called  the  Utica  epoch,  and  the  rock  itself  the  Utica 
shale,  from  the  city  of  that  name.  The  shale  is  very  car- 
bonaceous. This  is  due  to  the  presence  of  much  organic 
matter  and  gives  to  the  rock  its  dark  color.  The  Utica 
shales  shade  gradually  up  in  central  New  York  into  the 
coarser,  often  sandy  and  widely  distributed,  beds  of  the 
Hudson  *  epoch,  so  called  from  extensive  displays  along 
the  Hudson  River.  Thus,  as  we  had  deeper  and  clearer 
waters  in  the  earlier  part  of  the  Lower  Silurian  period,  so 
now  the  change  is  in  the  reverse  order.  We  must,  however, 
now  notice  the  important  fact  that  the  Hudson  rocks  of  the 
old  eastern  shore  lines  are  sandy  and  thick,  while  in  the  re- 
mote offshore  regions  of  the  Ohio  and  Mississippi  Valleys 
they  are  thinner  and  are  limestones.  This  is  a  general  prin- 
ciple which  applies  to  many  epochs  and  their  deposits,  and  is 
a  conspicuous  illustration  of  the  manner  in  which  rocks  tell 
the  story  of  ancient  geography.  About  Cincinnati  700  feet 
of  shaly  limestones  of  Hudson  age  contain  abundant  fossils. 
According  to  Dana,  between  3,000  and  4,000  feet  of  Utica 
and  Hudson  shales  were  penetrated  by  a  boring  near 
Albany,  N.  Y. 

*  According  to  Clarke  and  Schuchert,  "  it  is  becoming  increasingly 
evident  that  the  great  mass  of  shale  in  the  Mohawk  and  Hudson  River 
Valleys,  which  was  designated  at  an  early  date  by  this  term,  is  resolv- 
able into  horizons  extending  from  the  Middle  Trenton  to  and  including 
the  Lorraine  beds."  The  term  Lorraine  is  taken  from  a  series  of  shales 
in  Jefferson  County,  N.  Y. 


FIG.  159.— Living  Hydrozoa,  to  illustrate  the  ancient  graptolites. 


FIG.  160.— Diplograptus  quadrimucronatus.    Complete  free-swimming  colony,  Utica 
shale,  Herkimer  County,  N.  Y.— RUKDEMANN,  in  New  York  Reports. 


PALEOZOIC  ERA 


323 


LIFE  IN  THE  LOWER  SILURIAN  PERIOD 

Several  types  which  appear  in  moderate  numbers  in 
Cambrian  times  now  become  abundant.  This  is  particu- 
larly the  case  with  corals,  Crinoids,  and  mollusks.  Before 
the  end  of  the  period  we  shall  chronicle  the  entrance  of 
air-breathing  creatures  and  backboned  animals.  We  must 
here  also  briefly  describe  certain  kinds  of  fossils  which  first 
become  plentiful  in  Lower  Silurian  rocks. 

315.  Graptolites. — These  fossils  are  found  in  Cambrian 
rocks,  but  are  abundant  in  the  Lower  Silurian.  They  are 


FIG.  161.— Diplograptus  and  Monograptus. 

usually  seen  as  long,  narrow,  sometimes  branching  impres- 
sions on  the  bedding  surfaces,  and  hence  receive  their  name, 
which  means  pen  stones,  from  their  resemblance  to  a  quill 
pen.  On  one  or  both  sides  of  this  axis  or  stem  are  notches 


GEOLOGY 


or  cells,  each  one  of  which  was  occupied  by  an  individual 
of  the  little  community,  all  of  which  had  a  common  body  in 
a  central  tube.  Some  of  the  forms  are  shown  in  Fig.  161. 
There  were  many  species,  and  they  were  often  characteristic 
of  a  series  of  rocks.  Their  greatest  development  is  Lower 
Silurian,  though  they  continue  through  the  next  period. 


FIG.  162.— Cup  corals.    Single  polyps  and  a  community.    (These  are  Upper  Silurian 
forms.) 

316.  Corals. — These  important  rock-makers  also  occur 
in  Cambrian  formations,  but  are  first  abundant  here.  The 
Paleozoic  corals  were  of  a  considerably  different  pattern 
from  the  modern  forms,  although  all  have  a  general  resem- 
blance. The  principal  kinds  were  three — the  Cup  corals, 
the  Favosite  or  Honeycomb  corals,  and 
the  Halysite  or  Chain  corals.  In  the 
first  the  cups  or  polyps  were  solitary 


FIG.  163.— Favosite  coral ;  cross  section,  vertical  section,  and  general  v 


or  in  groups  or  bundles,  and  often  large,  even  to  a  foot  in 
length  in  extreme  cases,  and  such  having  diameters  of  two 


PALEOZOIC  ERA 


325 


or  three  inches.  The  inner  space  may  be  divided  by  radial 
partitions,  horizontal  floors,  or  by  irregular  partitions  forming 
a  mass  of  small  cells.  In  Favosites  the  polyps  are  small  and 
polygonal,  and  massed  together  in  great  numbers.  Their 
arrangement  in  Halysites  is  very  graceful,  and  is  sufficient- 
ly shown  in  the  figure  (Fig.  178).  The  corals  as  a  whole 
show  that  the  seas  in  which  coralline  limestone  was  mak- 
ing were  warm  and  of  moderate  depth.  Unlike  graptolites, 
the  corals  grow  in  importance  in  the  succeeding  period. 

317.  Crinoids. — These  are  so  named  from  their  likeness 
to  a  lily,  and  are  the  ancient  kindred  of  the  existing  star- 
fishes and  sea  urchins,  though  a  few  of  them 
have  been  found  living  to-day,  being  dredged 
from  the  ocean  bottom.  In  Paleozoic  times 
they  were  very  abundant.  They  possess  great 
symmetry  and  grace  of  form,  and  contributed 
largely  to  the  making  of  some  limestones. 
A  typical  Crinoid  has  a  spherical,  pear- 
shaped  or  urn-shaped  case,  called  the  calyx, 


FIG.  164.— Crinoid, 
showing  arms 
and  upper  part 
of  stem. 


FIG.  165.— Cystids,  one  showing  two  rudimentary  arms. 


which  is  made  of  hard  plates  and  holds  the  vital  parts. 
Rising  in  a  circle  above  this,  around  a  central  mouth, 
are  several  arms,  with  delicate  branches,  giving  them  the 


326  GEOLOGY 

appearance  of  plumes.  By  these  the  water  was  stirred  and 
food  conveyed  to  the  mouth.  The  whole  was  mounted 
upon  a  stem,  which  was  attached  (not  rooted)  at  the  sea 
bottom.  The  stem  varied  from  a  few  inches  to  several 
feet  in  length,  and  was  made  of  a  column  of  joints  like 
small  coins,  firmly  bound  together,  but  flexible  as  a  whole 
Forests  of  these  graceful  organisms  must  have  covered 
many  sea  bottoms.  A  more  primitive  kind  has  a  short 
stem  or  none  at  all,  plates  less  symmetrically  arranged, 
and  no  branching  arms.  These  are  known  as  Cystids,  and 
came  to  their  height  during  Lower  Silurian  time.  An- 
other sort  are  known  as  Blastoids,  or  bud-formed.  They 
are  indeed  bud-shaped,  having  a  fivefold  petalloid  arrange- 
ment. These,  like  the  typical  Crinoids,  culminated  later. 

318.  Brachiopods. — We  have  seen  that  these  forms  were 
numerous  in  Cambrian  times,  but  they  become  exceed- 
ingly abundant  in  the  Lower  Silurian  period.  The  Trenton 
waters  teemed  with  them,  as  some  mollusks  populate  the 
seas  of  to-day.  Among  the  most  noteworthy  we  name  the 
Lingula,  already  described ;  the  Discina,  a  disklike  shell  of 
similar  small  size,  though  larger  in  later  periods;  the 
Rhynchotrema ;  the  Orthis,  a  shell  with  a  straight  hinge, 


FIG.  166.  Fio.  167. — Rhynchotrema  capax.    Hudson  River 

Orthie  Davidsonia.  group,  Frankfort,  Ky. 

and  having  first  and  last  many  species ;  and  the  Leptaena. 
Orthis  testudinaria,  or  the  shield-shaped  Orthis,  may  be 
singled  out  as  especially  common  in  Trenton  seas. 

319.  Mollusks. — These  animals  came  to  great  numbers 
in  Lower  Silurian  times,  in  all  their  classes,  bivalves,  uni- 
valves, and  chambered  shells.  Bellerophon,  a  trumpet- 


PALEOZOIC  BRA 


327 


shaped  Gastropod,  and  Pleurotomaria,  a  low-coiled  shell  of 
the  same  class,  are  common  in  the  Trenton  rocks.  The 
most  striking  addition  to  the  molluskan  fauna  is  the  vast 
number  of  Cephalopods,  an- 
cestors of  the  Nautilus  of  to- 
day. A  few,  like  the  Nauti- 
lus, are  coiled,  some  are  sim- 
ply curved,  but  the  majority 


FIG.  168.—  Orthoceras,  restored,  show- 
ing position  of  the  animal,  the  cham- 
bers, and  siphuncle. 


FIG.  169.— Ormoceras,  showing  cham- 
bers and  large  siphuncle. 


were  straight  and  many  belong  to  the  genus  Orthoceras,  from 
words  meaning  straight  horn.  Some  had  the  diameter  of 
one's  finger  and  were  a  few  inches  in  length,  gently  tapering 
back  from  the  open  end  where  the  animal  resided.  Others 
were  several  feet  long  and  a  number  of  inches  in  diameter, 
up  to  a  foot  in  some  cases,  with  extreme  lengths  of  10  to  12 
feet.  Here  first,  then,  we  meet  with  animals  comparing  in 
size  with  creatures  of  modern  seas.  Like  the  Nautilus, 
these  ancient  shelled  Cephalopods  had  their  shells  divided 
into  compartments  by  cross  partitions,  through  all  of  which 
from  the  animal  backward  ran  a  small  tube  called  the 


328 


GEOLOGY 


siphuncle.  Good  specimens  often  show  siphuncle  and  par- 
titions. The  latter  were  set  on  a  simple  curve.  This 
should  be  remembered,  since  in 
later  periods  we  find  them  com- 
plicated in  high  degree,  illustrat- 
ing a  great  principle  in  the  evolu- 
tion of  life  on  the  earth.  A  pecul- 
iar class  of  mollusk,  classed  as  a 
Pteropod,  is  exemplified  in  Tren- 


FIG.  170.— Conularia. 


FIG.  171.— Pleurotomaria,  a  low-coiled  Gastropod. 


ton  rocks  by  Conularia,  a  four-sided  pyramidal  shell  of  con- 
siderable size.  The  reference  of  Conularia  to  the  Ptero- 
pods  is,  however,  doubted  by  some  authorities. 

320.  Crustacea, — The  Trilobites,  so  numerous  in  the 
Cambrian,  are  still  increasing  in  impor- 
tance. The  great  genera  are  as  follows  : 
Isotelus,  which  had  massive  head  and  tail 
pieces,  eight  segments  in  the  abdomen,  an 
elliptical  general  outline,  and  was  large, 
up  to  eight  inches  or  more  in  length ; 
Calymene,  smaller,  two  inches  long  or  less, 
distinctly  segmented  in  abdomen,  tail, 
and  middle  lobe  of  the  head,  and  often 
found  rolled  up.  Very  perfect  specimens 
occur  in  the  Trenton  of  New  York  and 
the  Hudson  rocks  of  Cincinnati ;  Trinu-  FIG.  irs.-Murchisonia, 
cleus,  small,  with  two  head  spines  extend- 
ing far  back,  and  a  prominently  lobed 
head ;  Triarthrus  Becki,  a  characteristic  and  very  abun- 
dant species  of  the  Utica  shale,  two  inches  in  greatest 


a  high-coiled  Gastro- 
pod. 


PALEOZOIC  ERA 


329 


length,  each  segment  of  the  middle  lobe  bearing  a  short 
spine.  Near  Eome,  N.  Y.,  specimens  have  been  found 
revealing  the  under  structures 
and  jointed  appendages  of  the 
Trilobite,  as  shown  in  Fig.  176. 
A  series  of  the  embryonic  forms  of 
this  species  has  also  been  found. 


FIG.  173. — Isotelus  maximni 
(Asaphus  platycephalus.) 


FIG.  174.— Calymene,  enrolled  specimen;  top  and 
side  view.    From  Lower  Silurian  of  Ohio. 


Lower  Silurian  rocks  also  contain  Leperditia,  a  small  crus- 
tacean beginning  in  the  Cambrian,  and  also  the  earliest 
known  examples  of  the  Bar- 
nacle and  Eurypterus. 

321.  Insects  and  fishes. — 
The  earliest  known  insect  is 
reported  from  the  Lower  Si- 
lurian of  Europe,  but  the 
first  known  American  exam- 
ple is  Upper  Silurian.  In 
1892  the  present  director  of 
the  Government  Geological 
Survey  found  fragments  of 
fishes  in  rocks  of  Trenton 
age  near  Cation  City,  Col. 
Thus  these  forms  began  much 
earlier  than  had  been  sup- 
posed. In  the  same  way 
fresh  discoveries  may  increase  the  known  antiquity  of  many 
other  branches  of  the  animal  kingdom. 


FIG.  175.— Trilobite,  Trinucleus ;  short 
body,  long  spines,  prominent  lobes  of 
the  head.  Enlarged. 


330 


GEOLOGY 


322.  Plants.— A  land  plant  is  also  reported  from  the 
Lower  Silurian  of  Great  Britain,  though  considered  doubt- 


ful by  some.  The  life  of  Lower  Silurian  times  was  abun- 
dant both  in  variety  and  in  number  of  individuals.  The 
number  of  species  of  Brachiopods,  Mollusks,  and  Trilobites 


PALEOZOIC   BRA 


331 


aggregated  several  thousands,  and  beds  of  limestone  many 
feet  thick  are  often  almost  wholly  made  up  of  two  or  three 
kinds  of  shells. 

323.  Economic  products  of  Lower  Silurian  rocks. — The 
Trenton  limestone  is  often  locally  used,  as  in  New  York 
State,  for  building.  A  black  fine-grained  bed  of  the  same 
in  eastern  New  York  takes  a  high  polish,  and  has  been 
known  as  Glens  Falls 
"  marble."  Some  of 
the  true  marbles  of  the 
Green  Mountains,  and 
of  the  red  and  varie- 
gated marbles  of  Ten- 
nessee, are  Lower  Silu- 
rian. Much  of  the 
Trenton  is  also  burned 
for  quicklime.  In 
northern  Illinois  we 
have  the  Galena  lime- 
stone of  Trenton  or 
Utica  age  as  the  source 
of  lead  ores.  In  Ohio, 
Indiana,  and  central 
New  York  the  Trenton 
limestone  is  the  great 
source  of  the  supplies 
of  natural  gas.  In 
northern  Ohio  and  In- 
diana the  Trenton  is 
reached  at  a  depth  of  a  little  more  than  1,000  feet.  The 
gas  has  been  produced  by  decomposition  of  the  original  or- 
ganic matter  of  the  rocks  ;  it  is  stored,  according  to  Orton, 
in  beds  of  Trenton  which  have  become  porous  through  sec- 
ondary changes,  and  is  held  there,  until  released  by  borings, 
by  an  overlying  mass  of  fine  close-textured  Utica  shale. 
Pressures  of  300  to  600  pounds  per  square  inch  have  been 


FIG.  177. — A  Khizocarp,  a  marine  plant  of  the 
Lower  Silurian  period. 


332  GEOLOGY 

observed,  but  much  greater  than  this  near  Baldwin sville, 
N.  Y.,  where  pressures  of  1,400  pounds  to  the  inch  have 
been  measured. 

324.  Soils. — The  making  of  these  is  doubtless  the  most 
important  economic  use  of  Lower  Silurian  limestones,  and 
of  the  highly  calcareous  and  carbonaceous  shales  of  the 
Utica  epoch.      The  productiveness  and  prosperity  of  the 
"  Blue  Grass  Region  "  of  Kentucky  are  due  to  its  substruc- 
ture of  Trenton  limestone,  while  in  regions  like  New  York, 
the  Trenton  of  the  Adirondack  regions,  and  north  of  Lake 
Ontario,  has  been  widely  mixed  with  the  soils  by  south- 
ward movements  of  glacial  currents. 

325.  Close  of  the  Lower  Silurian  period. — In  eastern  North 
America  we  now  have  important  geological  changes.     Much 
of  New  England  east  of  the  Adirondacks  and  New  York 
Highlands  had  been  covered  by  the  sea,  receiving  sediments 
during  various  Lower  Silurian  epochs.     In  the  vicinity  of 
Albany,  north  and  south,  there  was  free  passage  into  the 
waters  from  the  interior  sea  that  extended  over  central 
New  York  and  thence  westward  and  southward.     As  the 
period  was  closing,  the  thick  sea  border  sediments  of  the 
New  England  region  were  crumpled  and  uplifted  to  form 
the  Taconic   Range,  including  the  Green   Mountains   of 
Vermont.     The  disturbance  was  felt  as  far  north  as  Nova 
Scotia  and  southward  to  Virginia.     We  know  the  age  of 
the  range  because  Lower  Silurian  beds  were  folded  in  to 
form  it,  and  Upper  Silurian  beds  only  reach  eastward  to 
the  base  of  the  mountains,  and  lie  unconformably  on  their 
upturned  edges  in  eastern  New  York.     Accordingly,  sev- 
eral series  of  Upper  Silurian  rocks  thin  out  and  disappear  in 
central  New  York  going  eastward.     Thus  we  see  that  some 
of  the  marbles  and  crystalline  schists  of  western  New  Eng- 
land are  of  the  same  age  as  the  unchanged  Trenton  of  New 
York  or  the  Mississippi  Valley.      In  southern  Ohio  and 
eastern  Kentucky  the  strata  are  bent  into  a  broad,  low 
arch,  whose  axis  runs  in  a  nearly  north  and  south  direc- 


PALEOZOIC  ERA  333 

tion.  Some  observers  believe  that  this  arch  is  due  to  the 
mountain-making  pressures  with  which  the  Lower  Silurian 
Period  closed.  If,  however,  an  island  was  thus  formed,  it 
was  later  submerged,  for  younger  formations  were  deposited 
in  the  region.  These  younger  beds  have  for  the  most  part 
been  removed  by  denuding  forces,  and  thus  we  now  find 
interesting  exposures  of  Lower  Silurian  rocks,  rich  in  fos- 
sils, about  Cincinnati. 


CHAPTER  XIX 
PALEOZOIC  ERA 

UPPER  SILURIAN  PERIOD 

THE  rocks  of  this  period,  as  of  the  one  before  it,  were 
studied  by  Murchison,  and  were  named  by  him. 

326.  Epochs  of  the  Upper  Silurian. — The  development 
of  our  continent  was  similar  to  its  progress  in  Lower 
Silurian  times.  Five  epochs  are  distinguished  in  the  New 
York  formations : 

5.  Waterlirae  and  Tentaculite  Epoch  (Rondout 
and  Manlius). 


UPPER  SILURIAN 
PERIOD. 


4.  Salina  Epoch. 
3.  Niagara  Epoch. 
2.  Clinton  Epoch. 
1.  Medina  Epoch. 


The  rocks  of  the  Medina  epoch  are  named  from  Medina, 
N.  Y.,  and  consist  of  sandstones  and  shales.  They  are 
several  hundred  feet  thick  in  western  New  York,  covering 
a  belt  on  the  south  shore  of  Lake  Ontario,  and  they  form  the 
lower  part  of  the  gorges  of  Niagara,  and  the  Genesee  at 
Rochester.  They  are  not  found  westward  beyond  eastern 
Ohio.  In  central  New  York  there  are  beds  of  building 
stone,  known  as  the  Oneida  Conglomerate  ;  and  similar  beds, 
the  Shawangunk  Grit,  form  the  Shawangunk  Mountains 
west  of  the  Hudson.  Medina  sandstones  are  1,800  feet 
thick  in  Pennsylvania,  and  their  upturned  and  denuded 
edges  form  the  great  succession  of  mountain  ridges  which, 
334 


PALEOZOIC  ERA  335 

in  zigzag  courses,  inclose  the  valleys  and  overlook  the  low- 
lands. They  continue  to  Virginia  and  Tennessee.  More 
nearly  than  any  series  yet  studied  in  this  review,  their  out- 
crop represents  the  shore  line  of  the  interior  sea.  Hence 
we  do  not  find  them  far  away  from  the  old  shores,  as,  for 
example,  in  the  central  Mississippi  region. 

Overlying  these,  and  outcropping  south  and  west  where 
undisturbed,  are  the  rocks  of  the  Clinton  epoch.  They  are 
named  from  Clinton,  N.  Y.,  and  extend  westward  to  Wis- 
consin and  southward  to  Tennessee.  The  thickness  is  from 
80  to  1,000  feet,  and  the  character  variable — sandstones, 
greenish-gray  shales,  and  a  few  limestones,  with  one  or 
more  thin  beds  of  oolitic  iron  ore.  As  a  rule  they  were  laid 
down  in  shallow  waters.  They  lie  over  the  Medina  and 
under  the  Niagara  limestone  at  Niagara,  and  in  the  lower 
gorge  of  the  Genesee.  The  next  epoch  is  the  Niagara.  It 
it  is  one  of  the  great  limestone-making  intervals.  This 
is  equivalent  to  saying  that  the  waters  of  the  region 
where  Niagara  rock  is  now  found  were  not  beclouded  with 
land  waste,  and  were  of  some  depth.  It  appears  but  slightly 
along  the  Appalachians,  is  about  two  hundred  feet  thick 
in  western  New  York,  is  found  thence  to  Iowa,  and  occurs 
in  the  Black  Hills  of  Dakota.  It  is  one  of  the  most  exten- 
sive formations  of  the  State  of  Iowa.  Many  quarries  are 
opened  in  Niagara  limestone  in  Chicago,  and  the  channels 
of  the  Illinois  and  Mississippi  Rivers  and  of  the  Drainage 
Canal  are  excavated  in  this  formation. 

The  rocks  of  the  Salina  epoch  consist  of  red  and  green 
marly  shales,  and  associated  drab  limestones,  which  afford 
hydraulic  cement.  Hence  these  upper  beds  are  sometimes 
called  the  Water-lime  group.  The  distribution  of  the  Salina 
is  similar  to  that  of  the  Niagara  and  Clinton,  but  it  is  not 
known  in  the  far  West.  The  rocks  represent  a  period  of 
shallow  and  often  quiet  waters,  in  which,  by  evaporation, 
much  rock  salt  was  formed.  The  Waterlime  formation  is 
so  called  from  the  hydraulic  limestone  afforded  by  it,  and 


336 


GEOLOGY 


extensively  quarried  at  Rondout,  N.  Y.  The  Tentaculite 
limestone  has  been  so  named  from  its  common  fossil.  It 
may  better  be  called  the  Manlius  limestone,  from  a  town  in 
central  New  York. 

In  New  York  the  Medina,  Clinton,  Niagara,  and  Salina 
rocks  thin  and  nearly  or  quite  run  out  to  the  eastward, 
though  most  of  them  reappear  going  south  into  Pennsyl- 
vania. None  of  the  Upper  Silurian  epochs  are  well  repre- 
sented in  the  Rocky  Mountain  region,  so  far  as  present 
knowledge  goes.  The  continental  evolution  which  went 
on  so  steadily  in  the  East  throughout  the  Paleozoic  era 
was  well-nigh  finished  in  respect  to  extent  of  land  before  the 
western  half  of  the  continent  fairly  entered  upon  its  growth. 
The  variety  of  geographic  conditions  represented  by 
Silurian  rocks  should  be  observed.  Thus  we  have  shore 

formations  in  the 
Medina,  shallow 
waters  in  much 
of  the  Clinton  and 
Salina,  and  lime- 
stone making  in 
the  Niagara,  Ron- 
dout, and  Manlius 
epochs. 

327.  Life  of  the 
Upper  Silurian  pe- 
riod.— Organic  de- 
velopment con- 
tinued along  the 
lines  followed  in 
the  two  preceding 
periods.  There 
were  no  abrupt  or  striking  introductions  or  extinctions. 
But  the  law  of  organic  unfolding  was  illustrated  by  the 
decline  or  disappearance  of  some  species  and  genera  and 
the  quiet  entrance  of  others  on  the  scene. 


FIG.  178.— Halysites  (Chain  coral). 


PALEOZOIC  ERA 


337 


Undoubted  land  plants  are  found  fossil  in  a  few  cases  in 
Upper  Silurian  rocks.  Their  rarity  does  not  prove  that 
land  vegetation  may  not  have  been 
common,  for  we  must  remember 
how  subject  to  decay  land  forms 
are.  The  Graptolites  decline  from 
their  culmination  in  the  previous 
period  and  become  nearly  or  quite 
extinct.  The  corals,  however,  nour- 
ish in  great  profusion,  forming  true  FlQ-  179.— spirifer  audacuius, 

i  »      .        ,i        -»T.  i  showing     interior     structure. 

coral  reefs  in  the  Niagara  epoch.      (This  species  is  Devonian.) 
The  Echinoderms  are  represented 

not  only  by  a  multitude  of  Crinoids,  but  by  the  forms  so 
abundant  in  modern  seas,  the  starfishes  and  sea  urchins. 
Brachiopods  continue  in  great  force,  and  there  are  some 


FIG.  180. — Block  of  limestone,  showing  numerous  Tentaculites. 

noteworthy  introductions  of  new  sorts,  such  as  the  Spiri- 
fer, the  Atrypa,  and  the  Pentamerus. 

The  Spirifers  are  a  noteworthy  genus  which  comes  in 
during  the  Clinton  epoch,  and  increases  in  the  number  of 


338 


GEOLOGY 


species  and  of  individuals  during  subsequent  epochs  and 
periods,  but  becomes  nearly  extinct  at  the  close  of  the  Pa- 
leozoic era.  They  carry  within  two  calcareous  spiral  coils, 
which  are  often  well  displayed  in  weathered  specimens  or 
artificial  sections  (Fig.  179).  The  form  of  the  shell  is 


FIG.  181.— Block  of  Niagara  shale,  showing  three  specimens  of  Calymene  Blumen- 
bachii.    At  the  top  is  the  caudal  shield  of  Homalonotus. 

various,  but  especially  triangular,  with  a  long  straight 
hinge,  and  sometimes  a  broad,  smooth,  curved  hinge  area. 
Several  species  from  the  different  epochs  will  be  shown 
in  the  figures  as  we  proceed.  The  Atrypa  is  another  genus 
which  continues  through  many  epochs  and  has  a  number  of 
species.  The  shell  is  nearly  circular,  and  often  an  inch  in 
diameter.  Commonly  one  valve  is  much  more  plump  than 
the  other,  and  sometimes  in  the  later  epochs  the  surface  is 
covered  with  spines. 


PALEOZOIC  ERA 


Pentamerus — so  named  from  a  fivefold  division  of  its 
interior — is  a  plump  shell,  one  to  two  inches  in  diameter, 
one  of  whose  valves  has  a  prominent  rounded  beak.  Cer- 
tain species  of  it  occur  in  the  Clinton  rocks.  It  must  not 
be  thought  that  these  are  all  the  important  Brachiopods  of 
these  ancient  seas.  Many  others  are  of  great  numbers  and 
interest,  but  in  an  elementary  work  only  a  few  of  the  most 
characteristic  forms  can  be  noticed.  A  sense  of  the  reality 
of  ancient  organisms, 
and  of  the  progress  of 
life  throughout  the 
earth's  history,  is  what 
the  student  should  win 
from  the  present  study. 

The  Mollusks  still 
yield  the  palm  to  the 
Brachiopods,  though 
all  the  classes  are  rep- 
resented. Pleurotoma- 
ria  and  Murchisonia 
continue  from  the 
Lower  Silurian,  and 
the  Avicula,  a  Lamelli- 
branch  with  a  winglike  extension,  is  not  uncommon.  A  new 
Pteropod  is  characteristic  of  the  Tentaculite  or  Manlius 
limestone,  a  small,  slender  shell  with  ribs,  shown  in  Fig.  1 80, 
and  often  covering  the  surfaces  of  the  beds.  The  Trilobites 
are  still  abundant,  though  not  so  conspicuous  an  element 
in  the  fauna  as  in  Lower  Silurian  times.  In  the  Waterlime 
or  Eondout  beds  are  many  remains  of  Eurypterus,  shown 
in  Fig.  182.  It  often  grew  to  a  foot  in  length.  Associated 
with  these  forms,  a  scorpion  was  found  some  years  ago,  near 
Waterville,  X.  Y.  It  has  the  great  interest  of  being  the 
first  air-breathing  creature  yet  discovered  in  American 
rocks,  though  two  or  three  of  equal  or  greater  age  have 
been  found  in  Europe.  A  few  fish  remains  are  found,  but 


FIG.  182. — Eurypterus  restored,  ventral  and  dorsal 
views. 


340 


GEOLOGY 


they  do  not  become  abundant  until  the  Devonian  times. 
An  account  of  the  early  fishes  is  reserved  for  the  chapter 
on  that  period. 

A  few  plant  remains  are  reported,  which  are  important 
as  representing  beginnings.    Certain  markings  or  networks 

of  stemlike  forms  are 
common  in  Medina 
rocks,  and  were  former- 
ly described  as  plants. 

328.  Economic  prod- 
ucts of  the  Upper  Silurian 
period. — As  in  most  rock 
systems,  building  stones 
are  furnished  by  some 
of  the  series.  Thus  Me- 
dina sandstones  are  con- 
siderably used,  especial- 
ly for  paving  purposes. 
Niagara  rocks  are  locally 
employed  for  building 
and  for  making  quick- 
lime. The  iron-ore  beds 
of  the  Clinton  epoch 
have  been  considerably 
worked  in  New  York 
and  other  States.  The 
ore  is  a  red  hematite, 
oolitic,  and  often  con- 
taining fossils.  The  most  important  product  of  American 
Upper  Silurian  rocks  is  common  salt.  It  must  not  be 
thought  that  salt  beds  are  peculiar  to  any  one  period. 
They  may  be  accumulated  at  any  time  when  shallow  basins 
of  sea  water  become  more  or  less  isolated  from  the  main 
ocean.  If,  in  addition,  the  land  keeps  a  stable  position  rel- 
ative to  sea  level,  and  little  land  waste  is  brought  in,  evap- 
oration may  go  on  in  these  natural  pans  until  beds  many 


FIG.  183.— A  Silurian  scorpion  (from  Scotland). 


PALEOZOIC  ERA 


341 


feet  thick  are  formed.  An  influx  of  mud,  as  during  a  flood 
or  storm  season,  may  leave  its  record  as  a  layer  of  shale  be- 
tween two  beds  of  salt.  Hence  it  is  that  salt  beds  of  Creta- 
ceous age  are  found  in  Louisiana,  brines  of  lower  Carbon- 
iferous age  in  Ohio  and  Michi- 
gan, massive  rock  salt  of  Triassic 
age  in  England,  and  especially 
in  Germany,  where  the  beds  are 
many  hundred  feet  thick. 

Such,  then,  were  the  condi- 
tions that  prevailed  in  central 
and  western  Xew  York,  in  parts 
of  Ohio  and  Ontario,  during  much 
of  the  Salina  epoch.  Through- 
out the  present  century,  and  even 
earlier  by  Indians,  salt  springs 
were  known  in  the  region  of 
Syracuse,  N.  Y.  For  many  years 
the  brines  obtained  by  borings 
have  been  evaporated  in  solar 
vats  and  by  boiling.  About  twen- 
ty years  ago  a  chance  boring  for 
oil  in  western  Xew  York  was  the 
means  of  finding  the  actual  salt 
beds  of  the  formation.  They  are 
sometimes  80  feet  thick  and  oc- 
cur along  the  Genesee  Eiver  and 
eastward  to  the  center  of  the 
State.  As  the  Salina  shales  dip  southward  beneath  the 
younger  rocks,  the  salt  is  found  at  varying  depths,  from  800 
feet  on  the  north  side  of  the  belt  to  3,000  feet  on  the  south. 
Beds  of  similar  age  are  found  near  Cleveland,  Ohio,  and  God- 
erich  in  Ontario,  showing  that  a  large  area  was  affected  by 
salt-making  conditions.  Gypsum,  both  crystalline  and  mas- 
sive, is  found  in  rocks  of  the  same  epoch.  It  is  in  workable 
quantities  and  is  extensively  ground  and  sold  as  a  fertilizer. 


FIG.  184.— Eock  salt  of  Salina  age, 
western  New  York.  Two-foot 
cube  and  smaller  mass  with 
natural  fracture.  Museum  of 
Colgate  University. 


CHAPTER  XX 

PALEOZOIC   ERA 

DEVONIAN  PERIOD 

329.  General  statement. — The  period  receives  its  name 
from  the  county  of  Devon  in  the  south  of  England,  where 
its  formations  are  typically  seen.     The  designation  is  uni- 
versally adopted.     The  Devonian  period  in  North  America 
succeeds  the  Silurian  in  a  quiet  manner,  without  mountain- 
ous upturnings,  and  it  is  characterized  in  the  East  by  a 
great  series  of  shales  and  sandstones.     It  marks  a  consider- 
able growth  of  land,  and  thus  paves  the  way  for  the  more 
nearly  continental  conditions  of  Carboniferous  times.     A 
great  increase  of  land  plants,  forming  forests,  and  the  wide- 
spread development  of  the  fishes,  are  the  important  biologi- 
cal changes. 

330.  Epochs  of  the  Devonian. — We  give  them  as  follows : 

f  5.  Cheraung  Epoch. 

4.  Hamilton  Epoch. 
DEVONIAN  PERIOD -I  3.  Corniferous  (Onondaga)  Epoch. 

2.  Oriskany  Epoch. 
[  1.  Helderberg  Epoch. 

In  this  classification  no  account  is  taken  of  certain  local 
formations  occurring  chiefly  in  the  State  of  New  York. 
Some  of  them  will  be  mentioned  in  the  appropriate  con- 
nection. The  earliest  epoch  of  the  Devonian  is  the  Hel- 
derberg,* whose  rocks  form  a  deposit  300  feet  thick  in  east- 

*  The  Helderberg  is  here  transferred  from  the  top  of  the  Silurian  to 
the  base  of  the  Devonian  series,  following  J.  M.  Clarke  and  other 
authorities. 


PALEOZOIC  ERA  343 

ern  New  York  and  extend  far  south.  They  are  unimpor- 
tant in  the  West,  but  are  found  in  the  Connecticut  and 
St.  Lawrence  Valleys,  and  in  northern  Maine  and  Nova 
Scotia,  showing  the  wide  sweep  of  Helderberg  waters  in 
the  East.  They  are  limestones  with  abundant  fossils. 
This  means  that  the  northeast  shore  of  the  Interior  Sea  was 
considerably  submerged  and  that  animal  life  nourished  in 
clear  and  quiet  waters.  Above  the  Helderberg  rocks  lies 
the  Oriskany  sandstone,  so  named  from  the  village  of  Oris- 
kany  Falls,  southwest  of  Utica,  N.  Y.  It  is  there  but  12 
feet  thick,  and  passes  abruptly  to  the  Lower  Helderberg 
limestone  below  and  the  Corniferous  limestone  above.  It 
consists  of  coarse  quartz  sand,  and  thus  shows  two  abrupt 
changes  of  deposit,  and,  like  the  Medina,  its  outcrop  repre- 
sents the  east  and  west  shore  line  of  the  Interior  Sea  in 
New  York  at  the  beginning  of  the  Devonian  time.  South- 
ward along  the  Appalachians  it  is  often  of  greater  thick- 
ness, as  in  Maryland  and  Virginia.  It  disappears  in  west- 
ern New  York,  but  occurs  in  Ontario  and  southern  Illinois. 

331.  The  Corniferous  *  epoch  is  so  named  because  of 
the  nodules  and  layers  of  flint  or  hornstone  which  its  rocks 
contain.     They  were  formed  by  the  solution  and  concentra- 
tion of  the  siliceous  matter  of  protozoans  and  sponges,  as 
is  proved  by  the  finding  of  such  structures  when  the  flints 
are  studied  under  the  microscope.     The  limestone  is  full 
of  fossils,  and  the  organisms  have  produced  small  quanti- 
ties of  the  mineral  oil  which  is  sometimes  found  in  the 
formation.     The  Corniferous  rocks  extend  westward  from 
New  York,  and  are  well  developed  in  several  States  of  the 
central  West  from  Ohio  to  Iowa  and  southward. 

332.  The  Hamilton  rocks  are  so  named  from  typical 
exposures  on  the  lands  of  Colgate  University  and  elsewhere 
in  the  town  of  Hamilton,  N.  Y.     They  consist  of  a  great 


*0nondaga  (Clarke  and  Schuchert)  is  a  better  designation,  referring 
to  extensive  displays  of  these  rocks  in  Onondaga  County,  N.  Y. 


344  GEOLOGY 

series  of  shales  and  shaly  sandstones,  1,200  feet  or  more  in 
thickness,  and  extending  east  and  west  in  a  belt  about  20 
miles  wide.  They  are  sandy  in  eastern  New  York,  but 
more  calcareous  going  westward,  where  the  water  was 
deeper  and  received  less  land  waste.  About  100  feet  of 
black  mud  rock,  called  the  Marcellus  shale,  lies  at  the  base 
as  a  kind  of  bed  of  passage  from  the  Cornif erous  limestones 
to  the  sandy  deposits  of  the  typical  Hamilton.  This  epoch 
is  extensively  represented  in  other  States  and  parts  of  the 
continent.  Its  beds  attain  a  maximum  thickness  of  near- 
ly a  mile  in  Pennsylvania,  and  are  known  westward  to 
Wisconsin,  and  in  all  the  States  which  border  the  Ohio 
Eiver. 

333.  The  Chemung  rocks  are  a  great  series  (sometimes 
several  thousand  feet  in  thickness)  of  shales  and  sandstones. 
In  central  and  western  New  York  the  lower  and  older  parts 
of  the  Chemung  are  locally  designated  as  Portage,  Ithaca, 
and  Oneonta.  The  typical  Chemung  lies  geologically  higher 
and  forms  a  surface  belt  along  the  southern  border  of  the 
State.  Eastward  a  great  column  of  sandstones — mostly 
barren  of  fossils — forms  the  Catskill  Mountains,  but  is 
believed  to  be  the  thick  shore. formation,  identical  in  time 
with  the  finer  and  more  fossiliferous  beds  farther  west. 
Chemung  rocks  have  even  greater  thickness  in  parts  of 
Pennsylvania.  The  "  black  shale "  is  a  thin,  widespread 
equivalent  of  the  Chemung  series,  made  in  the  bottom  of 
the  Interior  Sea  south  and  west.  Thus  in  almost  every 
instance  we  have  found  thick  and  often  coarse  fragmental 
beds  forming  in  the  East,  and  thin,  generally  calcareous 
beds  in  the  Mississippi  Valley  region. 

The  coarse  materials  of  the  East  were  brought  by 
streams,  which  then  pursued  a  westward  direction  from  the 
unknown  extent  of  land  that  then  lay  along  the  Appala- 
chian belt,  and  perhaps  considerably  eastward  over  the 
present  domain  of  the  Atlantic.  Land  waste  some  miles 
in  thickness  laid  on  a  sinking  sea  floor  from  New  York  to 


PALEOZOIC  ERA 


345 


Alabama  requires  some  adequate  body  of  land  for  its  deri- 
vation.    This  could  only  have  lain  to  the  eastward. 

The  students  must  not  forget  the  islands  which  occu- 
pied the  central  parts  of  the  Interior  Sea  in  the  region  of 
Cincinnati  and  southward.  About  these  as  well  as  along 
the  older  shores,  the  Silurian  and  Devonian  formations 
were  accumulating,  so  that  as  we  come  to  the  close  of 
Devonian  times  and  the  opening  of  the  Carboniferous,  we 
find  shallow  waters  and  nearly  inclosed  marine  gulfs  where, 
in  pre-Silurian  times,  there 
had  been  a  sea  open  far  to 
the  west. 

Considerable  Devonian 
formations  are  found  in  the 
western  region  of  the  United 
States.  The  chief  are  in  the 
Wasatch  Mountains,  in  the 
Grand  Canon  district,  in  Ne- 
vada and  California.  An- 
other belt  of  Devonian  rocks, 
correlated  with  the  Hamil- 
ton series,  occurs  in  the  val- 
ley of  the  Mackenzie  Eiver, 
and  southward  to  Manitoba. 
Others  still  occur  in  the 
eastern  border  region,  as  de- 
fined by  Dana — that  is,  the 
Gulf  of  St.  Lawrence,  and 
in  Nova  Scotia  and  New 
Brunswick. 

334.  Life  in  the  Devonian 
period.  —  No  mention  has 
hitherto  been  made  of  the 
Sponges,  some  specimens  of 

which,  however,  are  found  even  in  the  Cambrian,  and  they 
are  not  uncommon  forms  in   the  Trenton,  Niagara,  and 


FIG.  185.— Hydnoceras  Avoca,  a  Devonian 
sponge,  southern  New  York.— After 
HALT,  and  CLARKE. 


346  GEOLOGY 

Helderberg  limestones.  They  occur  in  the  Corniferous 
limestones  also,  and  their  siliceous  spicules  are  found  in 
the  nodules  of  flint.  Various  species  appear  in  the  Hamil- 


FIG.  186.— Cora),  Zaphrentis  Roemeri,  Helderberg  epoch. 


FIG.  187. — Coral,  Favosites  conicus,  Helderberg  epoch. 

ton  and  Chemung  epochs.     The  Chain  corals  were  never 
abundant  and  are  not  known  in  the  Devonian.     Thus  we 


PALEOZOIC  EEA  347 

record  another  extinction  of  an  organic  group,  as  we  shall 
so  often  have  occasion  to  do.  The  Cup  and  Honeycomb 
types  remain  in  great  force,  as  seen  in  the  Helderberg, 
Corniferous,  and  parts  of  the  Hamilton  series.  A  noted 
reef  of  these  ancient  Devonian  corals  gives  origin  to  the 
falls  of  the  Ohio  Eiver  near  Louisville. 


FIG.  188.— A  Devonian  Crinoid. 

335.  The  Cystids,  or  more  primitive  Crinoids,  have  dis- 
appeared and  the  Blastoids  begin  to  come  in.  The  true 
Crinoids  with  branching  arms  continue  to  be  numerous, 
except  in  the  later  Devonian.  The  Chemung  seas  of  the 
East  were  too  muddy  to  favor  their  growth.  The  student 
must  not  think  that  the  absence  of  a  group  from  the  rocks 
of  one  region  means  that  they  did  not  nourish  somewhere 
during  the  same  time.  They  maintained  their  course  in 
more  favoring  seas,  and  often  migrated  back  to  the  same 
region  in  a  later  epoch  or  period,  and  deposited  their  re- 
mains in  overlying  rocks.  The  starfishes  or  Asteroid 
group  of  Echinoderms  appear  to  have  begun  their  existence 
in  Lower  Silurian  seas,  and  have  thus  far  gained  no  great 


348 


GEOLOGY 


numbers,  but  highly  elaborated  examples  are  not  uncom- 
mon in  the  Hamilton  rocks. 

336.  The  Brachiopods  keep  their  large  place  as  a  Pale- 
ozoic type.     Some  genera  come  up  from  the  Silurian,  but  a 


FIG.  189.— Atrypa,  Spirifer,  and  other  Hamilton  fossils  in  association  upon  a  single 
slab. 

great  number  of  genera  and  most  species  are  new.  In  gen- 
eral they  show  a  greater  degree  of  ornamentation  than  the 
pre-Devonian  kinds,  or  more  elegance  and  variety  of  form. 
Frequently  they  are  equipped  along  the  hinge  line,  or  over 
their  entire  surface,  with  spines.  The  Lingula  and  Dis- 


PALEOZOIC  ERA 


349 


cina  continue  to  be  common  forms.  These  are  known 
as  inarticulate  Brachiopods — that  is,  without  bony  pro- 
jections or  processes  for  hinging  the  two  valves  of  the 
shell  together.  Of  the  hinged  or  articulate  Brachiopods, 


FIG.  190.— Productella  Boydi, 
Chemnng. 


FIG.  191.— Spirifer  mucronatus. 


the  Orthis,  Atrypa,  and  Spirifer  come  from  earlier  times, 
while  Productella  appears  for  the  first  time.  Pentam- 
erus  galeatus  is 
an  important  spe- 
cies of  the  Helder- 
berg  limestone. 
Two  or  three  spe- 
cies of  Spirifer 
will  be  especial- 
ly named.  Thus, 
a  large,  coarse- 
ribbed  species, 
Spirifer  arenosus, 
is  common  in  the 
Oriskany  sand- 
stone. A  thin 
form  with  long 
hinge  and  pointed 


FIG.  193.  —  Tropidoleptns  carinatus.     A  Hamilton 
Brachiopod.    Ventral  and  dorsal  views. 


extremities  is  ex- 
tremely charac- 
teristic of  the  Hamilton  series — viz.,  Spirifer  mucronatus. 
It  has  a  fanciful  resemblance  to  a  butterfly,  a  feature  which 
is  often  noticed  by  those  unacquainted  with  geology.  Spi- 


350 


GEOLOGY 


rifer  medialis  and  Spirifer  granulifera  are  other  Hamilton 
species.  Spirifer  disjunctus  belongs  to  the  Chemung,  but 

has  a  wide  range,  being 
found  in  Europe  as  well 
as  in  America.  Atrypa 
reticularis  is  common  in 
the  Hamilton,  as  in  sev- 
eral previous  epochs,  and 
Atrypa  axpera,  a  spinose 
form,  shows  well  the  in- 
creasing elaboration  of  the 

Devonian  shells.  Rensselaeria  ovoides,  egg-shaped,  as  the 
specific  name  indicates,  is  a  large  characteristic  form  of  the 
Oriskany  epoch. 


FIG.  194.— Leptasna  rhomboidalis. 


Orthonota  nndulata.  Grammysia  bisulcata. 

PIG.  195.— Characteristic  Middle  Devonian  (Hamilton)  Lamellibranche. 

337.  The  Mollusks  are  growing  in  abundance  both  of 
species  and  individuals,  thus  looking    toward    the  more 


PALEOZOIC  ERA  351 

modern  times  when  they  should  take  precedence  of  the 
Brachiopods.  Especially  do  the  Lamellibranchs  thrive  in 
the  sandy  waters  of  the  Hamilton  and  Chemung  epochs. 
Among  the  common  bivalves  of  this  class  in  the  Hamilton 


FIG.  196.  -Goniatites  Patersoni,  western  New  York. 

strata  we  find :  Pterinea  flabella,  about  two  inches  across, 
with  wings  and  the  surface  covered  with  coarse  ribs ;  Or- 
thonota  undulata,  a  narrow  form  with  a  straight  hinge,  and 
various  species  of  Grammy sia.  Nine  hundred  species  of 
Devonian  Lamellibranchs  have  been  described.  The  straight 
and  curved  shells  of  the  Cephalopods  are  found  as  in  pre- 
vious periods,  though  fewer  and  smaller,  but  the  coiled 
Cephalopods  make  a  new  advance  in  the  Goniatite  of  the 
Lower  Hamilton.  The  student  will  recall  that  the  cross 
partitions  of  the  Orthoceras  are  plain,  or  simply  curved, 


352 


GEOLOGY 


like  those  of  the  existing  Nautilus.  But  in  the  Goniatite 
they  are  crimped,  or  strongly  curved  back  and  forth,  as 
seen  in  Fig.  196.  This  is  the  ancestor  of  the  Ammonite 
type,  which  comes  to  its  height  in  Mesozoic  times.  Thus 
we  have  another  illustration  of  the  small  beginnings  and 
gradual  unfolding  of  the  great  types  of  life,  both  on  land 
and  in  the  sea. 

338.  Trilobites  are  still  numerous,  but  neither  they  nor 
the  Cephalopods  longer  rule  the  seas ;  Homalonotus  is  a 
long  and  large  form,  sometimes  8  to  10  inches  in  length, 
found  in  earlier  periods,  but  living  abundantly  in  the 
Hamilton  epoch.  Here  are  also  found  great  numbers  of 
the  Trilobite  Phacops  rana,  illustrated  in  Fig.  199.  But  few 
Trilobites  are  found  in  the  upper  Devo- 
nian rocks.  The  Crustacea  are  also 
represented  by  large  Eurypterids  and  by 


PIG.  197.— Homalonotus 
Dekayi,  Onondaga 
County,  N.  Y. 


FIG.  198.— Eye  of  Phacopg 
rana,  showing  lenses. 


FIG.  199.— Phacops  rana, 
a  common  Trilobite  in 
Hamilton  rocks,  Gene- 
see  County,  N.  Y. 


creatures  somewhat  like  the  lobsters  of  to-day.  Leperditia, 
a  small  crustacean  with  an  elliptical  shell  or  case,  is  com- 
mon in  some  beds  of  Helderberg  limestone.  Insects,  like 
cockroaches  and  dragon  flies,  become  common,  keeping  pace, 
according  to  a  general  law,  with  the  progress  of  land  plants. 
339.  The  great  advance  in  the  Devonian  period  is  in 
the  number  and  variety  of  its  fishes.  They  are  not  the 


PALEOZOIC  ERA 


353 


Teleosts,  or  fishes  with  bony  skeletons  of  to-day,  but  are 
strange  and  primitive  forms.     Instead  of  being  covered  by 


FIG.  301.— Holoptychius. 

flexible  scales  familiar  to  us,  they  were  often  incased  in  a 
strong  armor  of  large  bony  plates,  or  covered  by  an  integ- 
ument of  smooth,  hard  scales,  often  of  rhomboidal  shape. 


FIG.  202.— Ptericthys  restored. 


On  account  of  these  characters  the  former  are  called  Placo- 
derms  (plate  skin),  and  the  latter  Ganoids  (luster).     The 


354 


GEOLOGY 


Selachians  were  fishes  with  skeletons  of  cartilage,  like 
modern  sharks.  Some  ancient  fishes  had  sharp  teeth, 
others  a  pavement  of  hard  plates  for  crushing.  A  frequent 
character  also  was  the  unsymmetrical  tail,  the  spine  being 


FIG.  203.— Vertebrated  and  non-vertebrated  fish  tails. 

prolonged  through  one  lobe  of  the  tail  fin.  Several  of  these 
features  are  shown  in  Fig.  203.  Some  Devonian  fishes  were 
of  great  size,  notably  Dinichthys  (which  simply  means  ter- 
rible fish),  of  the  Ohio  Devonian,  said  to  have  a  length  of 
18  or  20  feet.  In  some  Devo- 
nian beds  of  Scotland,  Hugh 
Miller  and  others  have  found 
the  greatest  profusion  of  fossil 
fishes,  as  though  shoals  of 
them  had  been  suddenly  killed 
by  some  catastrophe. 

340.  For  the  first  time  in 
the  history  of  the  earth,  land 

plants  now  become  abundant.  There  were  both  herbaceous 
plants  and  trees,  mostly  of  ferns,  lycopods,  and  horsetails, 
belonging  to  the  flowerless  plants.  These  earliest  forests 


FIG.  204. — Wing  of  a  Devonian  insect. 


PALEOZOIC  ERA 


355 


of  the  globe  were  inhabited  also  by  insects  of  lower  types, 
but  some  were  of  large  size.  The  higher  flowering  plants 
were  still  absent,  and  were  so  to  remain  for  a  very  long 
period.  Xearly  inclosed  bays  and  lakes  of  brackish  or  fresh 
water  were  perhaps  common,  and  such  was  the  case  in  an 
important  way  in  Scotland  during  the  accumulation  in  such 
basins  of  thousands  of  feet  of  the  Old  Ked  Sandstone,  which 
is  believed  to  correspond  in  time  with  the  marine  shales 
and  limestones  of  Devon  and 
Cornwall,  the  typical  Devonian  of 
Great  Britain.  Geographic  condi- 
tions both  of  land  and  water  were 
becoming  varied,  and  these  were 
accompanied  by  correspondingly 
varied  types  of  animal  and  plant 
life. 

341.  The  North  American  con- 
tinent at  the  close  of  the  Devonian 
period. — It  must  be  remembered 
that  marine  rocks  can  not  form 
land  during  their  deposition,  but 
only  after  their  upheaval.  Such 
slow  uprising  in  far-distant  periods 
it  is  impossible  definitely  to  trace. 
The  limits  of  the  next  younger 
system  of  rocks  do  not  coincide 
with  the  sea  borders  of  the  follow- 
ing period,  because  we  can  not 
know  how  much  of  the  later  sedi- 
ments has  been  removed  by  ero- 
sion. Hence  all  maps  reconstructing  a  continent  at  a 
given  time  involve  more  or  less  of  conjecture.  But  their 
general  truthfulness  makes  them  instructive  and  useful. 
Dana  gives  such  a  map  representing  the  progress  of  the 
American  continent  at  the  time  to  which  we  have  now 
come.  The  Atlantic  shore  line  is  somewhat  eastward  from 


FIG.  205. — Cephalaspis  from  the 
Old  Red  Sandstone.  See  sec- 
tion 340. 


356 


GEOLOGY 


its  present  position,  from  New  England  southward.  The 
Appalachian  belt  of  land  does  not  extend  beyond  central 
Georgia  and  northeastern  Alabama.  Its  northwest  shore 
runs  from  northeastern  Pennsylvania  through  the  Vir- 
ginias and  Eastern  Tennessee.  The 
border  of  the  Interior  Sea  passed 
along  the  southern  border  of  New 
York  and  Lake  Erie,  and  then  ran 
around  most  of  the  southern  pe- 
ninsula of  Michigan,  south  of  which 
are  the  lands  of  the  Cincinnati 
Anticline  which  now  form  a  penin- 
sula joining  to  the  mainland  on 
the  northwest  in  northern  Indiana 
and  Illinois.  Thence  the  west- 
ern shore  line  extends  northwest 
through  northern  Iowa,  western 
Minnesota,  and  far  away  through 
Canada.  A  large  island  lies  in 
Missouri,  south  of  the  Missouri 
Eiver.  Of  the  details  of  the  sur- 
face and  of  the  river  systems  of 
the  time  we  know  nothing. 

342.  Economic  products  of  the 
Devonian  period.— Some  beds  of  the 
Corniferous  limestone  are  used  for 
building,  and  locally  also  the  Ham- 
ilton and  Portage  sandstones.  The 
so-called  North  River  flags  are 
quarried  from  Hamilton  beds  in 
eastern  New  York.  Slabs  of  great 
size  are  sometimes  obtained,  owing 
to  the  infrequency  of  joint  planes. 
The  same  is  true  of  Chemung  rocks 
in  some  parts  of  southern  New 

FIG.  206.— Devonian  ferns  from  l  . 

New  Brunswick.  York.     The  most  important  prod- 


PALEOZOIC  ERA 


357 


net  of  American  Devonian  rocks  is  the  petroleum  of  west- 
ern Pennsylvania,  southwestern  New  York,  eastern  Ohio, 
and  West  Virginia.  It  is  stored  by  Nature  in  what  are 
called  the  "  oil  sands,"  which  are  beds  of  porous  sandstones 


FIG.  207. — Devonian  forest  restored. 


of  late  Devonian  age.     The  productive  sandy  layers  in  a 
given  place  may  be  one,  two,  or  three  in  number,  at  vary- 
ing intervals  in  going  down.     A  "  sand  "  may  be  productive 
24 


358  GEOLOGY 

at  one  place  and  barren  at  another.  The  oil  is  supposed 
to  have  been  produced  by  the  decomposition  or  slow  dis- 
tillation of  organic  matter  in  still  lower  beds.  In  the 
earlier  days  of  the  oil  industry  spouting  wells  were  com- 
mon, owing  to  the  pent-up  gases  held  with  it  in  the  rocks. 
Now,  moderately  flowing  or  pumping  wells  are  the  rule.  It 
is  a  common  practice  to  "  shoot "  new  or  waning  wells — 
that  is,  to  explode  at  their  bottom  a  heavy  charge  of  nitro- 
glycerin,  by  which  the  rock  is  shattered,  channels  opened, 
and  the  flow  increased.  The  oil  is  piped  to  local  reser- 
voirs, much  of  it  is  worked  into  various  products  at  neigh- 
boring refineries,  and  much  is  carried  by  great  pipe  lines  to 
distant  cities. 

Many  limestones  and  shales  contain  a  little  oil,  which 
may  gather  on  the  surface  of  pools  or  streams,  and  thus 
by  deceptive  indications  lead  to  unprofitable  investments. 
Even  in  the  oil  region  the  geologist  can  determine  at  what 
depth  a  given  oil  sand  will  be  met  in  boring,  but  he  can 
not  say  whether  or  not  it  will  prove  productive.  Crude  oils 
vary  much  in  composition,  specific  gravity,  and  color. 


CHAPTER  XXI 
PALEOZOIC  ERA 

CARBONIFEROUS  PERIOD 

343.  THE  names  of  the  other  Paleozoic  periods  have  had 
some  connection  with  a  locality.     This  period  is  so  named 
from  the  abundant  carbon  stored  in  the  rocks  in  the  form 
of  coal.     As  the  conditions  for  making  rock  salt  may  recur 
a  number  of  times,  so  this  is  not  the  only  period  in  the 
earth's  history  when  coal  was  formed ;  but  here  we  find  the 
earliest  deposit  which  has  economic  value,  and  at  the  same 
time  it  is  by  far  the  most  abundant,  taking  the  lands  as  a 
whole.     Coal  being  the  most  important  mineral  product  of 
the  earth,  with  the  exception  of  iron,  careful  study  has  been 
devoted  to  most  areas  where  it  occurs,  or  is  supposed  to 
occur.     This  is  done  both  by  private  and  by  government 
enterprise.     An  incidental  result  is  the  abundant  knowl- 
edge of  a  purely  scientific  nature  which  we  have  of  the 
Carboniferous  times.     When  we  say  this,  however,  we  must 
remember  two  things :  first,  that  the  truth  of  science  has 
the  highest  educational  worth  in  itself ;  and,  second,  we 
never  know  how  soon  such  truth  will  have  value  in  the 
common  affairs  of  men. 

344.  Subdivisions  of  the  Carboniferous  period. — Three  sub- 
ordinate periods  are  distinguished — namely,  the  Early,  Mid- 
dle, and  Late  Carboniferous.     The  first  of  these  is  more 
commonly  called  the  Lower,  or  Sub-Carboniferous — a  desig- 
nation which  is  used  more  suitably  of  the  rocks  themselves 

359 


360  GEOLOGY 

than  of  their  time  of  deposit.  The  second  is  the  Carbon- 
iferous proper,  or  period  of  the  Coal  Measures.  By  coal 
measures  we  mean  the  rock  strata  with  which  the  coal  beds 
are  associated.  The  coal  itself,  as  we  shall  see,  is  small  in 
bulk  and  area  as  compared  with  the  rocks  which  contain 
it.  The  Late  Carboniferous  is  called  the  Permian,  from 
the  government  of  Perm  in  Russia.  It  is  often  considered 
as  a  distinct  period  following  the  Carboniferous,  but,  as  its 
formations  are  less  full  in  North  America  than  across  the 
seas,  it  is  better  in  this  elementary  study  to  retain  it  in 
a  subordinate  position.  We  shall  take  up  the  main  divi- 
sions of  the  Carboniferous  in  their  order.  We  must  first 
observe  that  New  York  has  now  become  mainly  a  land  sur- 
face, and  its  rocks  no  longer  furnish  a  scale  of  comparison. 
We  shall  distinguish  four  of  the  more  important  regions  of 
deposition : 

1.  The  Interior  Sea  of  the  East.     This  has  now  become 
a  "  double-headed  bay,"  reaching  to  northeastern  Pennsyl- 
vania on  the  one  hand,  and  over  southern  Michigan  on 
the  other.     A  great  peninsula  stretches  from  southern  Wis- 
consin southeast  over  the  region  of  the  Cincinnati  Anti- 
cline, and  thus   divides  these  secluded  waters  from  the 
region  of  the  Mississippi  Valley. 

2.  We  therefore  set  the  central  Mississippi  country  by 
itself,  and  find  in  it  a  typical  series  of  rocks. 

3.  The  Eastern  Border,  which,  as  emphasized  by  Dana, 
has  been  a  place  of  accumulation  throughout  earlier  Paleo- 
zoic times. 

4.  The  region  of  the  Rocky  Mountains  and  westward. 
345.  Early  Carboniferous  of  the  several  regions.— We  have 

seen  that  in  the  later  Devonian  epochs  the  Interior  Sea  in 
New  York  and  Pennsylvania  received  a  vast  deposit  of 
coarse  rocks,  while  west  and  south  the  beds  were  thin  and 
fine.  Denudation  seems  to  have  been  active  around  the 
head  of  the  great  gulf.  There  perhaps  were  the  highest 
mountains,  the  largest  lands,  and  the  most  powerful  rivers. 


PALEOZOIC  ERA  361 

The  same  conditions  held  on  there  in  the  Early  Carbonifer- 
ous times.  Two  formations  were  made  in  Pennsylvania. 
The  Pocono  sandstones  and  conglomerates  form  much  of 
the  plateau  surface,  especially  in  northern  Pennsylvania. 
They  are  followed  and  overlain  by  the  softer  Mauch  Chunk 
shale.  These  subdivisions  correspond  in  importance  with 
those  of  the  epochs  already  studied.  Their  formations  pass 
into  limestone,  as  we  go  southwest  to  Virginia,  Tennessee, 
and  Alabama. 

The  corresponding  rocks  of  Ohio  are  called  the  Waver- 
ley  group,  and  in  Michigan  the  Marshall  group. 

In  the  Mississippi  region  we  have  another  succession  of 
deposits.  They  are  all  limestones  and  hold  the  greatest 
profusion  of  marine  fossils.  They  cover  large  areas  in 
Indiana,  Illinois,  Iowa,  and  Missouri.  The  names  Kinder- 
hook,  Osage,  St.  Louis,  and  Chester  are  applied  to  the  sev- 
eral groups  or  series  of  rocks  and  to  the  epochs  during  which 
they  were  formed.  Sandstones,  shales,  and  limestones  of 
Early  Carboniferous  age  are  distinguished  in  Nova  Scotia 
and  Xew  Brunswick.  As  there  is  no  coal  in  the  Carbonif- 
erous system  of  the  far  western  region,  its  subdivisions  are 
not  so  well  known,  and  indeed  may  not  in  any  great  degree 
correspond  with  those  of  the  East. 

346.  Coal  measures.— We  take  first  those  of  the  Eastern 
interior.  Here  Pennsylvania  becomes  the  typical  State, 
both  because  the  formations  occur  on  a  grand  scale  and 
because  the  geological  surveys  of  the  State  have  during 
many  years  been  carried  to  a  high  degree  of  perfection. 
The  first  geological  survey  was  executed  by  the  brothers 
H.  D.  and  W.  B.  Rogers,  and  the  second  survey,  under  Les- 
ley, has  issued  more  than  100  volumes,  covering  all  coun- 
ties of  the  State  and  exhibiting  the  coal  formations  to  the 
fullest  degree. 

A  great  series  of  coarse  rocks,  known  as  the  Pottsville 
Conglomerate,  forms  the  foundation  of  the  coal  measures  in 
the  East.  Then  succeed  shales,  sandstones,  limestones,  and 


362  GEOLOGY 

beds  of  coal,  alternating  with  each  other  in  almost  any 
order.  Taking  a  given  bed  or  "  seam  "  of  coal,  it  may  be  a 
few  inches  or  several  feet,  or  rarely  40  to  50  feet  thick,  and 
is  commonly  found  in  the  following  associations  :  Below  is 
a  clay,  often  known  as  the  fire  clay,  which  frequently  con- 
tains stumps  and  roots  of  the  ancient  trees.  Above  lies  the 
coal,  which  is  a  true  bed,  though  often  called  a  seam,  or 
incorrectly  a'  vein.  Like  other  sedimentary  rocks,  it  will 
be  nearly  horizontal  unless  disturbed,  in  which  case  it  may 
stand  at  any  angle.  The  coal  may  be  solid  through- 
out, or  may  have  interbedded  thin  layers  of  shale  or 
"bone."  These  may  become  so  numerous  as  to  unfit  the 
bed  for  working.  Above  the  coal,  forming  the  roof  in 
mining,  is  another  shale,  often  packed  with  ferns,  leaves, 
stems,  or  other  remains  of  plants.  They  mark  the  time 
when  fresh  earthy  sediments  invaded  the  coal-making 
swamps  and  buried  the  last  growth  of  vegetation.  Above 
this  shale  may  come  a  sandstone,  a  mass  of  shale,  or  a  lime- 
stone, or  a  succession  of  these,  which  may  be  thin  or  thick, 
and  may  have  been  laid  down  in  a  fresh-water  lake,  a  brack- 
ish-water bay,  or  comparatively  open  sea,  depending  upon 
the  amount  of  subsidence  and  the  surrounding  geographic 
conditions.  What  these  conditions  were,  and  what  the 
succession  of  geographic  changes  was,  we  are  now  prepared 
to  understand. 

By  sedimentation,  or  by  the  general  uplift  of  the  grow- 
ing continent,  a  sea  area  becomes  a  broad  marsh  or  a  region 
of  swamps  and  lowlands.  The  climate  is  warm  and  moist, 
and  the  forest  develops  in  great  luxuriance  with  the  smaller 
ferns  and  other  undergrowths.  In  these  moist  areas  the 
vegetation  is  preserved  and  accumulates  like  the  peat  of 
present  times.  After  a  greater  or  less  period,  for  the  mak- 
ing of  a  thick  or  thin  carbonaceous  layer,  subsidence  en- 
sues, the  waters  come  in  and  the  vegetable  layer  is  covered 
by  mud,  perhaps  of  a  delta,  fine  or  coarse,  making  shale  or 
sandstone  according  to  the  topography  and  rocks  of  adja- 


PALEOZOIC  ERA  363 

cent  lands,  or  a  greater  sinking  carries  the  region  some- 
what offshore,  the  waters  become  clear,  marine  life  nour- 
ishes, and  limestones  are  made.  After  a  time  fresh  eleva- 
tions, or  continued  deposit,  bring  the  bottoms  near  to  the 
surface  again,  and  the  coal  swamps  are  renewed.  That  the 
student  may  better  appreciate  the  remarkable  way  in  which 
rocks  and  coals  are  thus  interleaved,  the  following  table  is 
given  nearly  as  reproduced  by  Dana  from  the  Pennsylvania 
Reports : 

Monongahela  River  Series 

Feet. 

Shale  and  sandstone 70-82 

Waynesburg  main  coal  bed 6 

Sandstone  and  shale,  60 ;  limestone,  5 ;  sandstone, 

20  ;  fire  clay,  3 88 

Uniontown  coal  bed 1-3 

Sandstone  and  shale,  60;  limestone  and  shale,  18; 

sandy  shale,  40 ;  limestone  and  shale,  55 173 

Sewickley  coal  bed 1-6 

Sandstone  and  shale,  25 ;  limestone,  18 ;  sandstone, 

10 53 

Redstone  coal  bed 1-6 

Shale,  sandstone,  and  limestone 50-62 

Pittsburg  coal  bed 5-12 

Fire  clay 3 

494 

It  will  be  seen  that  the  greatest  thickness  of  the  above 
series  is  494  feet,  and  that  there  are  5  coal  beds,  in  all 
from  14  to  33  feet.  Shales  and  clays  are  noted  as  occur- 
ring 10  times,  sandstones  7  times,  and  limestones  5  times. 
Probably  many  minor  alternations  are  not  recorded  in  the 
table.  Nothing  could  better  illustrate  the  freedom  of  all 
sedimentary  series  of  rocks  from  arbitrary  rules.  Each 
case  must  be  studied  by  itself,  and  the  wonder  is  not  that 
so  few  but  that  so  many  general  laws  can  be  ascertained 
after  long  courses  of  patient  investigation. 

In  order  to  explain  the  present  arrangement  of  the  coal 
beds  of  Pennsylvania,  we  must  for  a  moment  anticipate 


364  GEOLOGY 

the  mountain-making  which  closed  the  Paleozoic  era.  The 
rocks  and  coal  beds  of  eastern  Pennsylvania  were  thereby 
much  upturned  and  folded,  and  the  anthracite  fields,  like 
that  of  Scranton  and  Wilkesbarre,  lie  in  great  canoe-shaped 


FIG.  308.— Map  of  Pennsylvania  coal  regions. — After  LESLEY. 

synclinal  valleys,  while  immense  areas  of  rock  and  coal 
have  been  denuded  from  the  intervening  anticlinal  arches. 
The  anthracite  of  the  east  is  of  the  same  age  with  the  soft 
coals  of  western  Pennsylvania,  only  the  former  is  meta- 
morphosed by  mountain-making  pressures.  No  doubt  the 
two  coal  areas  were  once  continuous,  and  the  coal  fields  of 
Broad  Top  surviving  between  the  two  are  an  impressive 
proof  of  this.  The  same  great  coal  field  extends  in  an 
important  way  into  eastern  Ohio  and  southwest  through 
West  Virginia,  eastern  Kentucky,  and  Tennessee  into  Ala- 
bama. 

The  coal  measures  of  the  central  Mississippi  areas  also 
contain  two  great  coal  fields,  which  may  have  been  one 
until  their  continuity  was  destroyed  by  erosion.  The  east- 
ern area  lies  in  Indiana,  southern  Illinois,  and  western 
Kentucky;  the  western  is  larger,  extending  from  south- 


FIG.  209.— A  forest  of  the  Coal  period. 


366  GEOLOGY 

western  Iowa  over  parts  of  Missouri,  Kansas,  Arkansas, 
and  Indian  Territory,  into  Texas.  There  is,  besides,  a 
region  of  coal  measures  in  the  southern  peninsula  of  Michi- 
gan covering  7,000  square  miles.  They  were  formed,  as  it 
appears,  in  a  closed  basin,  and  with  the  lower  rocks  help  to 
form  a  succession  of  strata  like  a  series  of  gigantic  saucers 
of  diminishing  size. 

On  the  eastern  border  small  coal  areas  are  found  in  New 
England,  especially  in  Rhode  Island.  The  coal  is  a  hard 
anthracite,  sometimes  almost  like  graphite,  but  the  vegeta- 
tion is  typically  Carboniferous.  The  great  coal  deposits  of 
the  Eastern  Border  are  in  Nova  Scotia.  Seventy-six  dirt 
beds  are  recorded,  of  which  15  contain  deposits  of  coal. 
The  total  area  of  Carboniferous  coal  measures  in  North 
America  is  more  than  200,000  square  miles. 

347.  Carboniferous  formations  of  the    Rocky  Mountains 
and  Pacific  slope. — These  are  Carboniferous  only  in  name, 
and  are  so  called  because  belonging  to  the  period  of  coal 
accumulations  in  the  East.     The  extensive  soft  coals  of 
Colorado,  Wyoming,  Washington,  and  other  States  are  of 
later  age.     Carboniferous  strata  outcrop  in  Colorado  at  the 
eastern  foot  of  the  Front  Range.     In  the  Grand  Canon 
region  they  are  important,  and  are  called  the  Aubrey  sand- 
stone and  limestone.     Thirteen  thousand  feet  of  rocks  of 
this  period  occur  in  the  Wasatch  Mountains.     Strata  of 
this  time  are  also  found  in  the  Great  Basin  and  in  the 
Sierra  Nevada,  and  are  reported  by  the  Canadian  geologists 
from  a  number  of  arctic  localities. 

348.  Further  observations  on  the  coal  measures. — Here  we 
note  first  the  unquestionable  vegetable  origin  of  the  coal. 
It  is  shown  by  the  order  of  association  of  the  coal  with  its 
contiguous  beds,  soil  and  roots  below,  with   trunks  and 
branches  above.     It  is  also  proved  by  the  preservation  of 
vegetable  structure,  such  as  cells,  within  the  coal  itself, 
and  accessible  to  microscopic  study.     If  further  confirma- 
tion were  needed,  we  should  find  it  in  the  coal  series,  rang- 


PALEOZOIC  ERA  367 

ing  from  freshly  deposited  vegetation  through  all  grades 
of  peat,  lignite,  and  bituminous  coal,  to  anthracite  and 
graphite.  It  is  time  also  for  the  student  further  to  ponder 
upon  the  vast  period  of  time  needful  for  the  operations 
which  we  have  now  described.  We  have  seen  that  there 
were  76  successive  swamps  with  intervening  rock  forma- 


FIG.  210.— Vegetable  structure  in  coal  (as  seen  under  the  microscope). 

tion  in  Nova  Scotia.  More  than  100  of  these  alternations 
are  reported  from  South  Wales.  Every  foot  in  thickness 
of  coal  has  required  several  feet  of  fresh  vegetable  matter, 
and  the  oscillations,  of  which  there  were  so  many,  no  doubt 
went  on  as  slowly  as  such  changes  now  take  place  on  the 
New  Jersey  or  Scandinavian  shore. 

349.  Late  Carboniferous  or  Permian  rocks. — Only  a  gen- 
eral reference  is  needful  here.  About  1,000  feet  of  the 
upper  "  barren  measures  "  of  Pennsylvania  and  West  Vir- 
ginia are  of  this  age.  With  these  are  classed  certain  beds 
lying  over  the  coal  measures  from  Nebraska  to  Texas,  and 


368  GEOLOGY 

at  the  top  of  the  Carboniferous  system  in  the  Grand  Canon 
region  and  in  Nova  Scotia.  As  already  suggested,  the 
Permian  formations  assume  great  importance  across  the 


FIG.  211.— Group  of  Carboniferous  ferns. 

LIFE  IN  THE  CARBONIFEROUS  PERIOD 
350.  Plants. — We  have  seen  that  land  plants  begin  to 
appear  probably  as  early  as  Lower  Silurian  times,  and  that 
they  were  in  considerable  force,  even  making  forests,  in 


PALEOZOIC  ERA 

the  Devonian  period.  We  now,  however, 
see  them  in  great  prominence.  They  are 
a  large  part  of  the  living  world,  and  also 
form  the  culmination  of  Paleozoic  veg- 


369 


FIG.  212.— Foliage  of  coal  plants. 


FIG.  213.—  Calamites, 
resembling  modern 
horsetails. 


etation.     Some  of  the  chief  kinds  of  Carboniferous  plants 
form  but  a  small  part  of  the  world's  flora  in  all  later  periods. 

About  2,000  species  of  plants  are 
known  from  Carboniferous  rocks,  or 
about  one  fourth  of  all  reported  fossil 
plants.  And  this  great  display  of  vege- 
tation took  place  some  millions  of  years 
ago.  Several  hundreds  of  species  of  fern 
are  found,  many  of  them  preserved  with 
the  perfection  and  beauty  of  a  herbari- 
um of  modern  ferns.  Some  were  tree 
ferns,  like  some  still  found  in  tropical 
forests. 

To  the  class  of  Lycopods  belong  two 
very  important  sorts  of  Carboniferous 
trees,  which  form  a  large  part  of  the 
material  for  the  beds  of  coal.  One  is  FIG. 


I 


PIG.  215.— Lepidodendron.    A,  tree  restored  ;    B,  leaf ;    C,  cone  and  branch ;   D, 
branch  aiid  leaves  ;  I,  L,  M,  bark  with  leaf  scars. 


PALEOZOIC  ERA 


371 


the  Lepidodendron,  or  scale  tree,  so  called  from  the  dia- 
mond-shaped leaf  scars  arranged  in  spiral  order  and  cover- 
ing the  surfaces  of  trunk  and  limbs  (Fig.  215).  These 
trunks  were  sometimes  from  2  to  4  feet  in  diameter,  and 
the  trees  attained  a  height  of  more  than  50  feet. 


.  216.— Carboniferous  Blastolds. 

The   Sigillaria,  or  seal  trees,  had  trunks  marked  by 
fluted  columns,  and  a  vertical  row  of  leaf  scars  resembling 
a  seal  is  found  on  each  column.     The  trunks  were  mostly  a 
cellular  mass,  but  they  possessed  pith  and 
medullary  rays,  and  thus  combined  the  char- 
acters  of   modern   endogenous   and   exoge- 


FIG.  217.— Carboniferous  fruits. 

nous  plants.  This  is  an  important  fact,  for  it  shows  that 
in  the  plant  as  in  the  animal  world  the  ancestral  forms  often 
combine  features  of  structure  which 
are  found  in  separate  and  special 
types  to-day.  This  helps  us  to  think  FlG.  2*i8^ 
of  all  living  forms,  from  the  dawn  enlarged. 

of  life  until  now,  as  forming  a  family  or  evolutionary  tree. 
Another  class  which  has  its  greatest  examples  in  the 


372 


GEOLOGY 


Carboniferous  period  is  the  Equiseta,  or  horsetails  (Fig.  213). 
These  were  jointed  plants  like  the  common  diminutive  horse- 
tails now  growing  in  moist  places,  or  the  familiar  scouring 
rush.  The  stems  were  fluted,  and  bore  a  whorl  of  leaves  at 


FIG.  219.— A  Carboniferous 
Crinoid. 


G.  230. — A  Carboniferous  sea  urchin. 


each  joint.  They  attained  the  size  of  trees,  growing  to  a 
height  of  20  to  40  feet.  All  the  forms  thus  far  named  be- 
long to  flowerless  plants.  The  lower  orders  of  flowering 
plants  also  had  a  limited  development.  Thus  there  were 
conifers,  which  grew  on  the  higher  grounds,  and  whose 
nutlike  fruits  are  sometimes  found  as  fossils.  These  forms 
were  not  great  in  amount,  and  their  upland  situation  was 
not  favorable  for  preservation  to  future  ages.  Important 
changes  came  in  the  Permian  time,  especially  in  the  ap- 
proach to  extinction  of  the  Lepidodendrons  and  Sigil- 
larians. 

351.  Animal  life  of  the  Carboniferous  period. — AVe  now  for 
the  first  time  find  a  widely  distributed  and  well-preserved 
fossil  of  the  type  of  Protozoa.  It  has  about  the  size  and 
shape  of  a  grain  of  barley  or  wheat,  and  the  genus  is  known 
as  Fusulina.  It  occurs  in  many  Carboniferous  strata,  both 


PALEOZOIC  ERA 


373 


in  America  and  in  Europe.  The  student  must  not  suppose 
that  this  was  the  first  of  the  Protozoa.  When  it  is  remem- 
bered that  great  classes  of  these  forms  secrete  no  hard 
parts,  and  are  in  appearance  little  more  than  bits  of  jelly 
of  microscopic  size,  their  non-appearance  in  Paleozoic  rocks 
will  be  the  expected  and  not  the  surprising  fact. 

Corals  are  found,  though  more  commonly  in  early 
Carboniferous  times  than  afterward.  But  the  Favosite 
group  has  disappeared,  leaving  but  one  of  the  three  great 
types  of  Paleozoic  coral  in  existence.  Crinoids  had  an  im- 
mense development  in  the  deeper  seas  of  the  Early  Carbonif- 
erous, but,  as  we  should  expect  from  their  habit,  are  not 
abundant  in  the  coal  measures.  According  to  Dana,  650 


species  of  Crinoids  are  found  in  American  Lower  Carbonif- 
erous rocks.  Of  these,  more  than  350  species  occur  at  Bur- 
lington, Iowa.  Many  of  them  are  blastoids. 

Of  the  Brachiopods  the  Spirifers  are  still  plentiful. 
Some  of  them  are  very  large,  and  others  developed  curious 
and  almost  fanciful  shapes  in  their  shells.  Two  hundred 
species  of  Spirifer  are  reported  from  Devonian  and  Car- 
boniferous rocks,  but  they  are  few  in  the  Permian  part  of 
25 


374 


GEOLOGY 


the  period.  The  irregularity  of  some  of  the  Carboniferous 
forms  seems  to  be  a  prophecy  of  their  early  extinction. 
Another  great  genus  of  Carboniferous  Brachiopods  is  the 
spinose  Productus,  which  first  became  conspicuous  in  the 
Devonian  fauna.  Mollusks  begin  to  number  land  shells 
among  their  forms.  This  is  the  only  significant  change  in 
this  group.  But  three  species  of  Trilobites  are  now  left, 


-UTO^T^- 

FIG.  223.— Productus  Nebrascensis.    Three  views  of  a  specimen  from  the  coal  meas- 
ures of  Illinois. 

showing  how  near  to  extinction  this  great  group  of  Paleo- 
zoic creatures  has  come.  But  more  modern  forms  of  Crus- 
tacea are  multiplying.  Spiders  are  added  to  scorpions, 
which  had  been  so  long  present,  and  there  are  many  flies, 
locusts,  and  cockroaches.  There  were  none  of  the  higher 
insects,  as  there  were  none  of  the  higher  flowering  plants, 

rv 


rv 

rv 
il 

FIG.  223. — Goniatite,  two  views. 


FIG.  224.— One  of  the  last  of  the 
Trilobites. 


these  having  in  later  times  developed  together,  with  the 
greatest  modifying  influence  upon  each  other.  The  fishes 
are  still  of  the  ancient  types.  Amphibians,  which  are  now 


PALEOZOIC  ERA  375 

believed  to  have  appeared  in  Devonian  times,  are  somewhat 
common,  and  reptiles  begin  in  the  Permian. 

APPALACHIAN  EEVOLUTION 

352.  The  eastern  interior  has  been,  as  we  have  seen,  an 
area  of  deposits,  and  on  the  whole  of  growing  lands  since 
the  beginning  of  Paleozoic  times.  The  total  thickness  of 
the  several  Paleozoic  systems  of  rock  in  the  Appalachian 
belt  is  very  great— not  less  than  30,000  to  40,000  feet.  A 
great  range  or  series  of  parallel  folds  was  now  made,  ex- 
tending from  Catskill  on  the  Hudson  to  central  Alabama. 
Some  facts  about  this  range  were  given  in  the  chapter  on 
Physiographic  Geology.  Over  much  of  the  long  tract  of 
country  extending  from  the  Middle  States  almost  to  the 
Gulf,  the  mountains  were  of  Alpine  height.  West  of  the 
mountains  in  Pennsylvania,  West  Virginia,  and  other  States 
the  former  lowlands  were  elevated  into  a  plateau.  Up  to 
that  time  some  parts  of  the  region  had  still  been  liable  to 
incursions  of  the  sea.  The  possibility  of  this  now  passed, 
and  the  eastern  portion  of  the  continent  became,  for  the 
greater  part,  permanent  dry  land.  With  the  great  uplift 
denudation  became  extremely  active.  What  the  course  of 
the  drainage  was,  we  do  not  know.  It  has  been  surmised 
that  the  uplift,  extending  far  to  the  north  and  west,  may 
have  begun  or  caused  a  depression  along  the  general  course 
of  the  present  Great  Lakes  and  St.  Lawrence  Eiver,  or, 
roughly  speaking,  along  the  line  of  junction  of  the  Paleo- 
zoic with  the  earlier  formations.  But  if  this  is  capable  of 
proof,  it  has  not  yet  been  proved.  In  later  periods  some 
drainage  history  becomes  better  known.  It  is  most  im- 
portant to  emphasize  the  stupendous  transfer  of  materials 
which  now  began  to  take  place  from  the  great  mountain  re- 
gions, toward  the  east,  the  south,  and  the  west.  We  must  not 
forget  that  this  mountain  uplift  took  place  so  slowly  that 
the  rocks  affected  were  not  commonly  much  metamorphosed. 
Nevertheless,  geologically  it  was  rapid  and  is  entitled  to  be 


PALEOZOIC  ERA  37? 

called  a  revolution.  This  designation  is  suitable  for  an- 
other reason.  One  of  the  greatest  breaks  in  the  life  his- 
tory of  the  world  follows  it,  and  it  is  no  doubt  'due  in  large 
measure  to  the  change  of  conditions  and  enforced  removal 
of  species  from  their  former  habitats. 

The  Atlantic  shore  line  after  the  revolution  is  not  known. 
We  only  know  that  some  Mesozoic  waters  swept  up  to  the 
base  of  the  mountains  in  New  Jersey,  Virginia,  and  the 
Carolinas.  Dana  draws  the  post-Paleozoic  shore  line  a  little 
outside  of  the  present  Atlantic  and  Gulf  shores,  and  then 
westward  through  Texas,  eastern  Kansas,  and  along  the 
western  border  of  Minnesota.  "West  of  that  shore  was  sea, 
whose  islands  formed  a  skeleton  of  the  future  lands,  much 
as  they  had  in  pre-Paleozoic  days.  The  Kocky  Mountains 
and  Sierra  Nevada  were  sketched  in,  and  a  great  island  lay 
on  the  Nevada-Utah  border.  In  time  to  come  eastern 
North  America  was  to  be  a  region  of  land  sculpture  with 
deposit  on  its  fringe,  but  western  North  America  was  to  be 
a  theater  of  land-making  through  long  periods. 


CHAPTER    XXII 

MBSOZOIC  ERA 

TRIASSIC  AND  JURASSIC  PERIODS 

353.  THE  Mesozoic  era  is  a  great  natural  division  of 
geological  history.  It  is  based  primarily  upon  its  life, 
which  is  intermediate  between  the  ancient  and  modern 
groups.  Some  remnants  of  the  old  life  survive,  and  there 
are  abundant  prophecies  of  the  new,  but  the  era  has  its 
own  strong  characters.  It  is  often  called  the  Age  of  Rep- 
tiles, because  these  were  large  in  numbers  and  often  of 
huge  size.  Some  lived  in  the  water,  others  on  the  land,  and 
others  could  fly.  Thus  they  ruled  in  all  fields  of  existence. 
When  we  say  this,  however,  we  must  remember  that  other 
creatures  were  abundant  also.  Corals  and  mollusks,  and 
crustaceans,  echinoderms,  and  fishes  were  as  important  as 
ever,  though  we  may  not  always  be  able  to  find  the  strata 
which  preserve  them  ;  just  as  the  general  life  of  the  world 
did  not  stop  when  the  Napoleonic  armies  were  on  the  march, 
although  the  history  of  that  period  has  much  to  say  about 
them. 

In  geographical  progress  also  in  North  America  the 
era  stands  well  by  itself.  We  have  already  observed  that 
eastern  North  America  was  chiefly  the  product  of  Paleozoic 
times.  In  the  Mesozoic  era,  however,  with  the  exception 
of  a  strip  along  the  Atlantic  and  Gulf  coasts,  the  principal 
land-  and  mountain-making  was  in  the  West.  Triassic  and 
Jurassic  formations  were  first  studied  in  Europe.  They 
378 


MESOZOIC  ERA  379 

have  thus  far  been  found  less  typically  developed  in  Amer- 
ica, especially  in  the  East.  It  will  hence  be  better  for  us 
to  take  them  up  together.  The  life  of  the  two  periods  is 
also  sufficiently  a  unit  in  its  great  features  to  warrant  such 
a  treatment.  The  Triassic  period  is  so  named  from  a  three- 
fold grouping  of  its  formations  in  Germany.  The  Jurassic 
period  takes  its  name  from  the  great  succession  of  rocks 
and  fossils  in  the  Jura  Mountains,  which  are  outliers  of  the 
Alps  between  France  and  Switzerland. 

TRIASSIC  ROCKS  OF  EASTERN  NORTH  AMERICA 

354.  Students  and  teachers  who  live  in  the  vicinity  of 
these  formations  will  find  a  full  and  interesting  account  of 
them  in  a  Bulletin  (No.  85)  of  the  United  States  Geological 
Survey,  by  Prof.  I.  C.  Russell.  They  are  there,  as  often 
elsewhere,  called  the  Newark  Formation,  from  their  occur- 
rence about  Newark,  N.  J. 

They  lie  in  a  series  of  long  patches  in  a  belt  on  the 
southeast  side  of  the  Appalachian  Mountain  system,  and 
extend,  with  breaks,  from  Nova  Scotia  into  North  Carolina. 
They  consist  mainly  of  sandstones,  often  varying  into  beds 
of  conglomerate  and  shale,  or  less  commonly  into  limestone. 
They  do  not  contain  marine  fossils,  and  hence  could  not 
have  been  made  along  the  borders  of  the  open  sea.  They 
seem  to  have  been  deposited  in  hollows  or  basins  filled 
with  fresh  water,  or  in  great  estuaries  with  brackish  water, 
largely  protected  from  the  sea.  Very  coarse  conglomerates 
are  sometimes  found,  even  with  bowlders  two  or  more  feet 
in  diameter,  raising  the  question  whether  glaciers  may  not 
have  supplied  them  from  the  adjacent  mountains.  But 
there  is  no  conclusive  evidence  of  this.  We  must  remem- 
ber that  the  mountains  were  yet  young,  and  their  torrents 
were  powerful,  like  those  of  the  Alps  to-day.  The  materials 
in  the  beds  show  that  they  came  from  the  pre-Paleozoic 
crystalline  rocks  of  the  older  Appalachian  axis.  Hence  the 
drainage  of  the  Appalachian  folded  area  of  Paleozoic  rocks 


380 


GEOLOGY 


must  still  have  kept  its  original  northwest  direction.  The 
change  had  not  yet  come  by  which  the  Susquehanna,  Po- 
tomac, and  other  waters  should  descend  toward  the  At- 
lantic. 

During  the  period  of  deposition,  or,  as  some  think,  at  its 
close,  there  were  large  vol- 
canic  outflows  forming  the 
sheets  which  were  described 
244.     A  brief  ac- 


FIG.  236.— Track  of  Triassic  reptile  of  the 
Connecticut  Valley  ( x  J). 


count  of  the  principal  areas 
will  now  be  given.  The  Aca- 
dian area  lies  in  northern 
Nova  Scotia.  One  of  the 
most  interesting  lies  in  cen- 
tral Connecticut  and  Massa- 
chusetts, and  has  been  care- 
fully studied  for  many  years, 
especially  by  Hitchcock  and 
Emerson  of  Amherst,  Dana 
of  Yale,  Davis  of  Harvard, 
and  Percival  of  the  early  Con- 
necticut Survey.  The  rocks 
occupy  the  region  of  an  old 


bay  or  sound  which  extended 
from  the  sea  at  New  Haven  over  the  sites  of  Hartford  and 
Springfield  to  the  northern  border  of  Massachusetts.  On 
either  hand  lay  the  mountainous  uplands  built  largely  of 
Paleozoic  formations.  The  materials  of  these  older  rocks 
can  be  identified  in  the  younger  beds.  Probably  the  tides 
rose  and  fell  on  the  shores,  and  it  is  certain  that  land  rep- 
tiles, amphibians,  and  insects  dwelt  in  great  numbers  about, 
for  these  ancient  surfaces  have  yielded  thousands  of  speci- 
mens of  tracks,  formerly  thought  to  be  those  of  birds,  but 
now  known  to  be  mainly  reptilian.  Slabs  bearing  remark- 
able series  of  these  tracks  are  preserved  in  the  Amherst 
and  Yale  Museums.  Ripple  marks,  mud  cracks,  and  rain- 


MESOZOIC  ERA  381 

drop  impressions  prove  beyond  doubt  that  we  have  here  a 
splendid  display  of  ancient  shore  surfaces. 

The  Triassic  rocks  of  the  Connecticut  Valley  do  not  lie 
as  when  made,  but  dip  eastward  from  15°  to  25°.  They 
are  much  broken  by  faults,  so  that  their  thickness  is  not 
known,  but  may  be  5,000  to  10,000  feet.  Interleaved  be- 


FIG.  227.— Section  across  the  Triassic  rocks  of  the  Connecticut  Valley,  showing  trap 
sheets  (in  black),  faults,  and  outstanding  of  trap  after  denudation. — After  DAVIS. 

tween  the  strata  are  masses  of  lava.  The  prevailing  view 
seems  to  be  that  at  intervals  during  the  deposition  of  the 
beds,  sheets  of  lava  were  poured  out,  covering  more  or  less 
of  the  bottom  of  the  sound,  after  which  they  were  covered 
by  younger  sediments.  In  some  cases  intrusive  sheets  were 
formed.  They  all  shared  in  the  uplift  and  faulting  which 
came  later.  The  denudation  of  the  region  has  since  cut 
away  the  softer  sediments  and  left  the  outstanding  lava 
beds  to  form  Mount  Tom,  Mount  Holyoke,  and  other  emi- 
nences of  the  valley.  These  often  have  a  steep  slope  of 
columnar  lava  on  the  west,  and  share  the  more  gentle  dip 
slope  of  the  sandstones  on  their  eastern  face. 

So  many  main  features  are  repeated  in  the  several  areas 
that  the  rest  may  be  more  briefly  described,  although  the 
largest  is  yet  to  be  named.  This  is  called  the  Palisades 
area,  and  extends  from  Haverstraw  on  the  Hudson  through 
New  Jersey,  Pennsylvania,  and  Maryland  into  Virginia.  It 
is  350  miles  long.  The  Palisades  of  the  Hudson  are  formed 
by  the  eastern  outcrop  of  one  of  its  great  lava  beds.  Under 
it  by  the  river  and  over  it  on  the  west  slope  of  the  ridge 
are  the  shales  and  sandstones.  The  Watchung  Mountains 
near  Orange,  N".  J.,  are  of  the  same  origin.  The  dip  of  the 
rocks  in  this  area  is  westward.  Several  smaller  but  impor- 
tant areas  from  35  to  above  100  miles  long  lie  in  the  same 


382  GEOLOGY 

northeast  by  southwest  line  in  Virginia  and  North  Carolina. 
One  of  these  is  called  the  Kichmond  area.  The  coal  of 
some  of  these  southern  patches  will  be  noted  in  a  later 
section.  Little  is  known  of  the  barriers  which  must  have 
shut  out  the  sea  on  the  east  of  these  troughs  of  deposition. 

355.  Triassic  rocks  of  the  West. — Hocks  of  this  period 
are  found  in  western  Kansas,  extending  thence  into  north- 
ern Texas.     The  Black  Hills  of  Dakota  consist  of  a  pre- 
Paleozoic  core,  with  Paleozoic  and  later  strata  dipping  off 
under  the  plains  in  all  directions.     Triassic  beds  are  in- 
cluded in  the  series  here.     Along  the  base  of  the  Front 
Eange  in  Colorado,  Triassic  strata  make  part  of  the  forma- 
tions whose  broken  edges  are  turned  up  against  the  moun- 
tains.   The  slabs  and  pillars  of  the  Garden  of  the  Gods  are 
reckoned  as  Triassic.     We  have  here  part  of  an  extended 
series  known  as  the  Ked  Beds.     They  are  also  seen  in  the 
same  relative  position  as  one  crosses  the  Kocky  Mountain 
belt  and  comes  down  on  the  west  slope  in  Colorado.     Tri- 
assic rocks  form  the  Vermilion  Cliffs  of  the  Colorado  pla- 
teaus, and  are  found  in  western  Nevada  and  northern  Cali- 
fornia.    These  far  western  Triassic  rocks    contain    some 
limestone,  and,  unlike  other  American  Triassic,  are  marine. 

No  marine  Triassic  or  Jurassic  rocks  are  known  in  the 
Atlantic  or  Gulf  belts,  but  the  Cretaceous  or  later  Mesozoic 
beds  abut  against  the  pre-Mesozoic  formations.  This  shows 
that  during  Triassic  and  Jurassic  times  the  land  was  higher, 
the  Atlantic  shore  farther  east,  and  the  Gulf  shore  farther 
south.  Later  the  land  sank  and  Cretaceous  seas  with  their 
deposits  crept  to  the  base  of  the  highlands  in  New  Jersey 
and  southward. 

356.  Western  Jurassic  rocks.— As  in  the  case  of  the  Tri- 
assic, so  the  Jurassic  beds  outcrop  about  the  Black  Hills 
and  along  the  flanks  of  the  Eocky  Mountains.     We  have 
seen  that  a  large  area  of  Paleozoic  land  existed  in  the  Great 
Basin   region.     This  and  the  Eocky  Mountain  belt  were 
the  chief  centers  of  growth  for  the  western  lands.     On  the 


MESOZOIC  ERA  333 

east  of  the  Great  Basin  land  deposition  went  on,  and  Juras- 
sic outcrops  are  now  found  in  the  Wasatch  Mountains  and 
the  Colorado  plateaus.  The  region  of  the  latter  became  a 
great  secluded  sea  opening  on  the  south,  but  yet  to  receive 
a  vast  series  of  Cretaceous  and  younger  rocks.  On  the  west 
also,  along  the  then  existing  Pacific  border  in  eastern  Cali- 
fornia, deposition  went  on.  Jurassic  shales,  soon  (soon  in  a 
geological  sense)  to  be  elevated  to  form  part  of  the  Sierras, 
became  after  metamorphism  the  gold-bearing  slates  of  Cali- 
fornia. 

357.  Elevation  of  a  Sierra  Nevada  mountain  range.— This 
is  now  known  to  have  occurred  at  the  close  of  the  Jurassic 
period.     A  great  series  of  rocks  had  been  long  accumulat- 
ing on  a  sinking  sea  floor,  precisely  as  in  the  eastern  Inte- 
rior Sea  in  Paleozoic  times.     At  length  the  pressure  made 
itself  felt  in  crushing  and  folding,  and  we  have  now  three 
great  mountain  ranges  in  various  stages  of  development. 
The  Appalachian  was  finished,  save  as  it  should  be  affected 
by  denudation.     The  Kooky  Mountains  were  outlined,  but 
their  chief  growth  was  to  come  later.     The  Sierras  were 
well  under  way,  but  were  to  receive  a  vast  addition  to  their 
height  by  faulting  and  uplift  in  later  times.     Their  devel- 
opment at  the  close  of  Jurassic  time  is  sometimes  called 
the  Sierran  Eevolution.     It  is  also  believed  that  the  up- 
heaval of  the  Coast  Eange  of  California  was   coincident 
with  the  making  of  the  Sierras,  thus  outlining  for  the  first 
time  a  gulf  where  now  runs  the  Great  Valley  of  California. 

LIFE  OF  THE  TEIASSIC  AND  JURASSIC  PERIODS 

358.  Plants. — The   forests   were   losing   some   of  their 
ancient  characters,  but  had  not  yet  taken  on  a   modern 
appearance.     The  Lepidodendrons  and  Sigillarians,  which 
were  so  common  in  the  Carboniferous  period,  now  became 
insignificant.     But  the  higher  Cryptogams  are  still  repre- 
sented by  the  Ferns  and  Horsetails,  and  the  Gymnosperms 
furnish  abundant  examples  of  the  two  great  orders,  the 


PIG.  228.— Branch  and  fruit 
of  Conifer. 


FIG.  229.— Cone  of  Jurassic 
pine. 


FIG.  230. — Triassic  fern,  from  Richmond 
coal. 


FIG.  231.-A  living  Cycad. 


MESOZOIC  ERA 


385 


Conifers  and  Cycads.  The  Conifers,  as  we  have  seen,  came 
over  from  Carboniferous  times.  The  Cycads  are  palmlike 
in  appearance  but  not  in  structure,  and  form  the  most 
characteristic  element  in  Meso- 
zoic  vegetation.  As  yet  there 
were,  at  least  so  far  as  is  known, 
no  Angiosperms  or  higher  flower- 
ing plants. 


FIG.  232.— Leaf  and  stem  of  Jurassic  Cycad. 

359.  Animals. — As  a  rule,  American  and  European  Tri- 
assic  rocks  are  poor  in  fossils,  owing  to  special  conditions 
of  deposit  in  waters  more  or  less  disconnected  from  the  sea. 
Jurassic  life  is  much  more  abundant  both  in  western  Amer- 
ica and  especially  in  Europe,  where  marine  waters  were 
again  extensive.  Jurassic  rocks  in  some  localities  preserve 
many  Foraminiferan  and  Eadiolarian  shells  and  a  great  pro- 
fusion of  sponges.  It  will  be  remembered  that  we  have 
already  met  with  Foraminifera  in  the  Fusilina  of  Carbonif- 
erous strata.  Hydrozoa  are  known  to  have  lived,  from 
quite  perfect  impressions  of  jellyfish  found  in  the  lime- 
stones of  Solenhofen,  Bavaria,  and  now  to  be  seen  in  the 
museum  at  Munich.  Corals  also  are  numerous  in  the  Ju- 
rassic. One  after  another  the  Paleozoic  patterns  have  dis- 
appeared, and  these  forms  now  take  on  a  modern  structure, 
having  their  rays  or  radial  partitions  in  multiples  of  six. 


386 


GEOLOGY 


Several  strata  of  the  English  Jurassic  were  thickly  set  with 
them,  showing  that  these  northern  waters  were  still  warm, 

and  reefs  like  those 
of  Florida  and  the 
Bermudas  were  grow- 
ing there.  There 
were  more  than  two 
hundred  species  of 
British  Jurassic  cor- 
als. The  same  con- 
ditions were  found  in 

central  Europe,  in  the  region  of  the  Jura  Mountains,  and 
portions  of  the  future  area  of  the  Alps. 

We  must  again  note  the  entire  disappearance  of  the 
Paleozoic  Cystids  and  Blastoids,  and  the  change  of  the 
true  Crinoids  into  the  modernized  patterns.  The  Echi- 
noids,  or  Sea  Urchin  group  of  Echinoderms,  had  attained 
some  development  in  the  Paleozoic  era,  but  now  grew  in 


FIG.  233. — Triassic  starfish,  dorsal  (a)  and  ventral  (b) 
surface. 


FIG.  834.— Jurassic  sea  urchin. 

numbers  and  in  resemblance  to  existing  forms.  Some,  for 
example,  become  elongated  and  unsymmetrical,  with  mouth 
at  one  end,  instead  of  having  the  mouth  below,  as  in  the 
common  sea  urchin.  These  unsymmetrical  forms  belong 
to  the  Spatangoids. 

Many  changes  had  occurred  in  the  Brachiopod  group 
during  the  Paleozoic  periods.     On  the  whole,  they  had  be- 


MESOZOIC  ERA 


387 


come  less  conspicuous,  though  still  numerous  in  the  Car- 
boniferous period.  We  have  now  to  record  a  still  greater 
diminution  in  their  ranks.  The  Lingula,  Discina,  and 
Terebratula  were  all  Paleozoic  families  of  greater  or  less 
antiquity,  which  lived  in  Ju- 
rassic seas  and  still  live  to- 


FIG.  235.— Jurassic  oyster. 


FIG.  236.— Pecten,  Triassic. 


day.  But  there  were  no  more  Spirifers,  Atrypas,  Orthis, 
or  Productus,  of  which  such  countless  hosts  had  lived 
in  Paleozoic  days.  The  Brachiopods  have  steadily  waned 
through  the  millions 
of  years,  and  the 
mollusks  have  as 
steadily  grown,  but 
with  new  families, 
genera,  and  species. 


FIG.  237. -Trigonia,  Jurassic. 


FIG.  238.— Ceratites,  showing  the  suture  lines 
simply  crimped. 


Some  existing  genera  of  Lamellibranchs  now  appear, 
but  with  different  species  from  those  in  present  seas. 
Thus  there  are  several  kinds  of  oysters,  some  of  large  size. 
Other  forms  are  Pecten,  Astarte,  and  Trigonia.  It  will 


388  GEOLOGY 

give  some  idea  of  the  number  of  Gastropods  to  note  that 
nearly  1,000  species  lived  in  the  Jurassic  seas  of  Britain. 
The  most  conspicuous  feature  of  Jurassic  and  of  all  Meso- 
zoic  molluscan  life  is  the  group  of  Cephalopods.  There 
had  been  a  steady  decline  in  the  Nautiloid  group  of  straight 
and  curved  shells,  and  the  Ammonoid  group  were  on  the 
increase.  These  are  the  forms  characterized  by  increasing 
complexity  of  the  transverse  partitions.  This  begins  to 
show  in  the  Goniatite  of  the  Devonian,  is  more  fully  seen 

Ammonite. 


0 
© 


NautUolds. 


FIG.  239.— Growing  complexity  of  sutures  in  Cephalopods.    Position  of  siphnncle 
shown  at  the  left. 

in  the  Ceratites  of  the  Triassic,  and  comes  to  its  height  in 
the  Jurassic  Ammonite,  where  there  may  be  three  or  four 
sets  of  coarser  and  finer  crimpings  in  the  edge  of  the  par- 
tition where  it  joins  the  outer  wall,  or  at  the  "  suture  "  line. 
By  stripping  off  the  outer  shell  this  crimped  edge  comes 
to  view.  This  series  of  more  simple  and  more  elaborate 
forms  is  of  great  interest,  not  only  because  seen  in  passing 
from  older  to  younger  strata,  but  because  it  marks  the  path 
of  development  of  the  elaborate  forms,  from  the  embryo  to 


MESOZOIC  ERA 


389 


the  mature  state.  The  individual  in  its  growth  displays 
the  same  succession  as  is  manifested  by  the  ancestral  series 
in  its  development  through  long  periods.  Embryologists 
and  paleontologists  have  found  that  this  is  a  great  principle 
of  organic  life,  and  it  becomes,  therefore,  one 
of  the  most  impressive  proofs  of  the  origin  of 
the  various  patterns  of  animals  and  plants  by 
development  from  ancestral  forms. 

These  Jurassic  Ammonites   are  in  great 
number  of  species  and  variety  of  ornamenta- 
tion.    Hundreds  of 'species  are  recorded,  and 
some  are  of  large  size,  even  two  or  three  feet 
in  diameter  in  a  few  cases.     This  high  state  of 
development  seems  to  be  the 
precursor    of    decline,     for 
they  disappear  by  the  close 
of  the  Mesozoic  era.      But 
the  naked  Cephalopods,  or 


FIG.  240.— Jurassic  Ammonite. 


FIG.  241.— A  living     FIG.  242.  —  Belem- 
cuttlefieh.  nite,  internal  shell. 


those  without  external  skeletons  or  shells,  were  already 
coming  in,  the  squids  and  cuttlefishes,  to  dominate  the 
group  in  later  and  modern  times.  Some  of  these  fossil 


390 


GEOLOGY 


forms  are  called  Belemnites,  from  a  rounded  and  elon- 
gated internal  rod  or  bone.  They  possessed  ink  bags, 
like  the  modern  cuttlefish.  These  are  sometimes  pre- 
served in  remains  of  the  ancient  species.  It  is  an  often- 
repeated  but  nevertheless  interesting  observation  in  text- 
books that  drawings  of  fossil  forms  have  been  made  with 
their  own  ink.  The  ancient  shelled  Cephalopods  are  called 


FIG.  243.— Jurassic  Crustacean. 


FIG.  244.— Jaw  of  Triassic 
amphibian. 


Tetrabranchs,  and  the  modern  naked  forms  are  Dibranchs, 
from  their  having  respectively  four  and  two  gills. 

Of  the  Crustaceans  there  are  no  more  Trilobites,  and 
modern  crabs  and  lobsters  begin  to  come  in.  The  Insect 
group  also  becomes  modern  in  fullness  and  variety,  includ- 
ing most  of  the  orders.  A  reported  specimen  of  Butterfly 
is  considered  doubtful,  the  more  because  none  of  the  Angi- 


MESOZOIC  ERA 


391 


osperms  or  higher  flowering  plants  have  yet  been  found  in 
rocks  as  old  as  the  Jurassic. 

360.  Vertebrates. — Here,  with  the  exception  of  the  great 
host  of  Ammonites,  we  find  the  chief  features  in  the  life 
of  the  times.  Thus  the  highest  animal  type  for  the  first 
time  becomes  supreme  in  the  world's  life,  a  position  which 


FIG.  245.— Jurassic  Ganoid,  with  view  of  scales  enlarged. 

by  one  or  another  of  its  classes  it  has  never  ceased  to  hold. 
The  Mesozoic  character  of  the  era  is  seen  in  its  fishes,  which 
still  number  the  old  Ganoids  and  sharks,  but  some  of  these 
approach  in  character  the  modern  Teleosts,  or  fishes  with 
bony  skeletons.  Amphibians  grew  in  numbers  and  size 
from  Carboniferous  times,  some  of  the  Triassic  forms  being 


FIG.  246. — Jurassic  sea  reptile,  Ichthyosaurus. 

large  and  powerful  creatures,  quite  unlike  the  insignificant 
modern  examples.  One  specimen  has  been  described  whose 
skull  had  a  length  of  two  feet.  About  50  species  of  sea  rep- 
tiles are  known  from  the  Jurassic.  Two  great  types  only 
are  here  named.  One  is  Ichthyosaurus  (fish  lizard).  It  was 
sometimes  30  to  40  feet  long,  with  a  relatively  large  head 


392  GEOLOGY 

set  close  upon  its  body,  having  long,  powerful  jaws  and 
many  sharp  teeth.  The  eyes  were  enormous,  the  tail  finned, 
and  its  limbs  were  two  pairs  of  short,  stout  paddles.  It 
quite  realizes  all  traditions  of  horrid  and  vicious  sea  mon- 
sters. Twenty-five  species  of  these  creatures  are  known 
to  have  lived  in  the  waters  of  Britain.  Nearly  50  species  also 
of  Plesiosaurus  are  there  found  fossil,  and  the  museums  pos- 
sess some  highly  perfect  skeletons.  The  members  of  this 
group  were  more  slender,  but  having  a  short  tail  and  longer 


FIG.  247. — Jurassic  sea  reptile,  Plesiosaurus 


neck,  and  larger  paddles  also.  The  head  was  light,  and  the 
creature  was  fitted  to  rear  and  dart  after  its  prey.  Its 
greatest  length  was  25  to  30  feet. 

The  land  reptiles  were,  if  possible,  more  wonderful  still, 
in  size  and  habit.  Some  were  by  far  the  largest  animals  of 
all  time  in  length  and  bulk.  They  are  given  the  general 
name  Dinosaurs,  which  means  terrible  lizard  or  reptile. 
Many  lived  on  vegetation,  but  others  were  carnivorous. 
In  comparison  with  their  size  their  heads  were  very  small. 
In  some  cases  the  nervous  masses  in  their  posterior  parts 
for  the  control  of  their  huge  limbs  were  many  times  larger 
than  their  brains.  Many  had  powerful  and  heavy  hind 
limbs  and  tails,  with  light  fore  limbs  as  well  as  slender 


MESOZOIC   ERA  393 

necks  and  small  heads.  This  fitted  them  to  move  in  part 
or  altogether  on  two  legs,  or  two  legs  and  the  tail.  Thus 
the  herbivorous  kinds  could  readily  reach  up  and  crop  or 
haul  down  leaves  and  boughs  of  trees.  More  strange  still, 


FIG.  248.— Jurassic  land  reptile,  Camptosaurus  ( x  B\,>,  Wyoming. 

the  light  fore  parts  and  bipedal  locomotion  give  many  of 
these  creatures  a  distinct  resemblance  to  the  birds,  and 
support  the  general  conclusion  to  which  naturalists  have 
come,  that  the  birds  and  the  reptiles  have  developed  from 
a  common  ancestral  stem.  It  will  be  useful  to  add  a  few 


FIG.  249.— Land  reptile,  Brontosaurus  ( 


facts  as  to  the  size  of  some  of  these  animals.  Bronto- 
saurus was  sometimes  60  feet  long.  The  Atlantosaurus, 
found  in  the  upper  Jurassic  rocks  of  Colorado,  had  a  length 
of  70  to  80  feet.  A  single  thigh  bone  was  6  feet  long. 


394  GEOLOGY 

A  single  vertebra  of  another  species  had  a  diameter  of  2 
feet.  Stegosaurus  is  remarkable  for  a  series  of  huge  bony 
plates  mounted  along  the  back.  Thus  we  have  immense 
size,  a  low  grade  of  intelligence,  with  fantastic  and  exag- 
gerated structures  as  somewhat  common  characters.  In 


FIG.  250. — Jurassic  land  reptile,  Stegosanrns  ( x  ^),  Wyoming. 


some  way  flowing  from  these  facts,  it  may  have  come  that 
the  group  lacked  stability.  They  perished  with  the  era  to 
make  way  for  higher  and  nobler  types. 

A  reference  to  their  geographic  distribution  should  be 
added.  Most  of  the  larger  tracks  of  the  eastern  American 
Triassic  are  those  of  Dinosaurs,  notwithstanding  their 
three-toed  impressions,  a  fact  which  again  is  of  interest,  be- 
cause they  were  formerly  thought  to  have  been  made  by 
birds.  It  is  somewhat  surprising  that  the  tracks  should  be 
found  in  thousands  in  a  region  which  has  yielded  none 
of  the  skeletons.  On  the  other  hand,  the  Jurassic  beds  of 
Colorado,  Wyoming,  and  Montana  have  yielded  a  great 
harvest  of  Dinosaurian  bones,  which  have  been  found  and 
amply  studied  by  Marsh,  Leidy,  Cope,  Scott,  and  other 
American  scholars. 


MESOZOIC  ERA  395 

As  if  these  curious  creatures  were  not  enough  to  give 
individuality  to  the  time,  we  find  also  flying  reptiles,  to 
which  the  name  Pterosaurs  has  been  given.  These  were 
small,  but  had  a  remarkable  combination  of  bird  and  rep- 
tilian characters.  The  bones  were  hollow  like  those  of  a 
bird,  and  the  head  was  light  but  large,  and  the  jaws  armed 
with  teeth.  There  were  no  feathers,  but  the  finger  occu- 
pying the  place  of  the  little  finger  was  greatly  lengthened, 
and  between  it  and  the  body  and  rear  limbs  stretched  a 
membrane  serving  as  a  wing.  Sometimes  there  was  a  long, 


FIG.  251.— Plying  reptile,  Pterodactyl. 


slender  tail  ending  in  a  flap.  The  lithographic  limestones 
of  Bavaria  have  afforded  examples  that  leave  no  doubt  of 
the  nature  of  this  creature. 

The  Pterosaur  is  a  birdlike  reptile.  The  next  step  is  a 
reptilian  bird,  and  these  have  been  found  in  the  same 
locality  with  a  reptilian  head,  feathered  wings,  and  a  verte- 
brated  tail,  with  feathers  on  either  side  of  it  to  the  end 
(Fig.  252). 

We  have  seen  that  many  forms,  as  the  Trilobite,  numer- 
ous Brachiopods,  and  types  of  coral,  passed  out  with  the 
Paleozoic.  Some  typically  Mesozoic  forms,  as  Cycads  and 


396 


GEOLOGY 


reptiles,  are  at  their  height.     We  come  now  to  a  modest 
but  significant  prophecy  of  the  modern  periods  of  the 


earth,  the  beginnings  of  the  mammals.     A  few  remains  of 
very   small  and  lowly  creatures    of  this  class  have  been 


MESOZOIC  ERA 


397 


found  both  in  Triassic  and  Jurassic  strata  of  America  and 
Europe.  Their  great  interest  lies  in  the  fact  that  we  have 
here  the  first  examples  of  the  group  to  which  man  and  all 
the  higher  animals  belong. 

361.  Economic  products.— Several  substances  are  found 
in  Triassic  and  Jurassic  strata  which  have  well  served  the 
needs  of  man.  Triassic  sandstones  form  most  of  the  "  brown- 
stone  fronts  "  in  Xew  York  and  other  cities  of  the  East. 
They  have  for  many  years  been  quarried  at  Portland,  Conn., 
East  Long  Meadow,  Mass.,  and  in  other  places.  They  ap- 
pear well,  but  are  not  especially  durable  when  set  as  small 
pillars  and  window  bases,  or  in  other  exposed  positions. 
The  "Peachblow"  sandstones  seen  in  some  of  the  build- 
ings of  Colorado  College  are  from  the  "  Bed  Beds "  of 
Triassic  age  in  that  State.  Several  references  have  al- 
ready been  made  to  the  remarkable  deposit  of  fine-grained 
limestone  of  Jurassic  age  at  Solenhofen  in  Bavaria.  It  is 
almost  unique  in  its  perfection  for  lithographic  reproduc- 
tion. 

The  Triassic  coal  beds  of  Virginia  and  North  Carolina 
are  of  considerable  value,  though  their  importance  has  been 
obscured  by  their  nearness  to  the  vast  deposits  of  Carbon- 
iferous times.  Eight  coal  beds  occur  in  the  Eichmond 
area,  and  some  are  of  considerable  thickness.  Thin  beds 
of  coal  of  more  geological  than  economic  interest  occur  in 
the  Triassic  of  Germany  and  the  Jurassic  of  Great  Britain. 

In  both  these  countries  are  found  beds  of  rock  salt 
of  the  same  period.  By  the  upturning  or  fracturing  of 
Jurassic  shales  in  the  Sierra  Nevada  Mountains,  veins  of 
quartz  were  formed  which  are  rich  in  gold.  Thus  these 
shales  became  the  "  auriferous  slates  "  of  California.  The 
Placer  deposits  (page  240)  are  produced  by  the  erosion  of 
these  slates,  and  the  veins  themselves  have  yielded  enor- 
mous values  of  the  precious  metal. 


CHAPTER    XXIII 

MESOZOIC  ERA 
CRETACEOUS  PERIOD 

362.  General  observations. — This  period  takes  its  name 
from  the  chalk  which  constitutes  important  strata  in  some 
countries,  especially  in  England,  where  the  name  was  first 
applied.     The  most  conspicuous  changes  in  the  living  in- 
habitants of  the  world  were  the  growth  of  Angiosperms, 
including  forest  trees  of  modern  kinds,  and  the  presence  of 
Teleosts,  or  bony  fishes,  in  the  seas.     It  is  usual  to  reckon 
two  divisions  of  time,  as  determined  from  the  succession  of 
rocks,  an  early  and  a  late,  Cretaceous  period.     The  areas  of 
deposit  in  different  parts  of  Xorth  America  were  so  re- 
mote from  each  other  and  so  separated  by  land  masses,  that 
geologists  have  had  trouble  in  making  out  equivalent  beds. 
As  is  usual,  the  strata  have  been  given  local  names,  and  so 
we  have  one  set  of  designations  for  the  Cretaceous  rocks  of 
the  Atlantic  coastal  region,  others  for  the  Gulf  region  east 
and  west  of  the  Mississippi  River,  a  different  set  for  the 
country  between  the  Mississippi  River  and  the  Rocky  Moun- 
tains, and  still  another  for  the  Pacific  coast. 

363.  Geography  of  the  Cretaceous  period.— The  student 
will  best  get  a  general  notion  of  what  happened  in  Creta- 
ceous times  if  he  at  once  takes  account  of  a  great  down- 
sinking  and  uprising  which  affected  most  of  the  continent. 
During  early  Cretaceous  time  the  land  was  high,  the  shores 
receded,  and  there  were  areas  of  fresh  water  along   the 
present  belt  of  Atlantic  lowlands  and  east  of  the  Rocky 


MESOZOIC  ERA 


399 


Mountains.  The  exception  to  this  was  over  Texas  and 
parts  of  Indian  Territory,  Kansas,  and  2s  ew  Mexico,  where 
a  marine  bay  stretched  northward  from  the  Gulf  of  Mexico. 
Before  the  middle  of  the  later  Cretaceous  the  sea  on  the 
east  had  swept  up  to  the  base  of  the  highlands  in  Xew  Jer- 
sey and  the  States  lying  southward,  as  shown  by  marine 
Cretaceous  rocks  now  found  there.  Thence  the  shore  line 


FIG.  253. — North  America,  showing  probable  land  areas  during  the  Cretaceous  sub- 
mergence.   Water  areas  shaded. — After  LE  CONTE. 

swung  around  to  the  west  and  crossed  the  middle  of  Georgia 
and  Alabama  and  retreated  along  the  present  valley  of  the 
Mississippi  to  southern  Illinois,  whence  it  stretched  south- 
westward  into  Texas.  The  area  of  the  Delta  and  of  hun- 
dreds of  miles  of  the  present  flood  grounds  of  the  Missis- 
sippi was  then  occupied  by  an  arm  of  the  sea.  Farther  west 
the  open  sea  again  swept  through,  as  is  believed,  from  the 
Gulf  to  the  arctic  regions,  and  westward  beyond  the  west 


400  GEOLOGY 

boundary  of  Colorado.  The  Kocky  Mountain  nucleus  was 
again  reduced  to  a  group  of  islands  as  in  Paleozoic  times. 
The  lands  of  the  Great  Basin,  however,  stretching  north 
into  British  America  and  south  into  Mexico,  still  separated 
this  interior  sea  from  the  Pacific  Ocean. 

As  the  continent  slowly  came  up  again  during  the  later 
part  of  the  period,  the  great  Western  Interior  Sea  was 
narrowed  and  made  shallow,  the  connection  between  the 
Gulf  and  the  arctic  seas  was  interrupted,  lakes  of  fresh 
water,  swamps  and  bays  with  brackish  water,  took  the  place 
of  the  ocean,  and  vast  quantities  of  vegetable  matter  were 
formed  in  the  marshes  of  this  closing  or  Laramie  epoch. 
This  was  the  great  coal-making  time  in  the  western  United 
States.  But  simple  emergence  was  not  all.  The  way  was 
preparing  for  the  Rocky  Mountain  revolution,  which  oc- 
curred at  the  close  of  the  Cretaceous  period,  and  thus 
marks  off  both  Cretaceous  and  Mesozoic  time  from  all  that 
follows  in  American  geological  history.  Here  is  included 
the  elevation  of  the  Rocky  Mountain  range,  from  Colorado 
far  northward  and  southward,  and  of  the  high  and  massive 
Wasatch  Range,  between  the  Great  Basin  on  the  west  and 
the  Colorado  plateaus  on  the  east.  There  were  extensive 
volcanic  eruptions  also  at  various  points  in  this  great  belt 
of  disturbance.  It  is  possible  to  determine  clearly  the 
time  of  this  mountain-building.  Cretaceous  deposits  with 
marine  fossils  are  found  at  an  altitude  of  10,000  feet  upon 
the  mountains.  The  region  must  have  been  below  the  sea 
level  until  after  the  strata  were  made.  The  disturbance 
was  therefore  post-Cretaceous.  But  the  succeeding  rocks 
of  the  early  Tertiary  period  lie  unconformably  against  the 
upturned  strata  of  the  mountains,  and  have  not  suffered 
disturbance.  The  movement  was  therefore  pre-Tertiary. 

The  continent  was  beginning  to  take  on  its  modern 
form.  The  Appalachian,  Sierran,  and  Rocky  Mountain 
revolutions  have  passed,  although  uplift  and  denudation 
have  yet  much  to  do  with  their  final  form.  The  Eastern 


MESOZOIC  ERA  401 

Interior  Sea  was  obliterated  with  the  Carboniferous  coal 
swamps  and  the  Appalachian  upheaval,  and  the  Western 
Interior  Sea  disappeared,  save  for  some  great  fresh-water 
lakes  after  the  Laramie  Coal  epoch  and  the  Eocky  Moun- 
tain upheaval.  The  West  and  East  were  joined,  and  the 
heart  of  the  lands  was  not  to  be  again  invaded  by  the  sea. 

364.  Cretaceous   peneplain. — Physiographers   and  geolo- 
gists who  have  given  attention  to  the  subject,  nearly  all 
believe  that  a  great  peneplain  was  developed  in  the  Eastern 
United  States  during  Mesozoic  times,  coming  to  an  ad- 
vanced stage   of   development  in   the    Cretaceous  period. 
Allusions  have  already  been  made  to  parts  of  this  surface 
in  the  chapter  on  Physiography.     Such  are  the  highlands 
of  western  New  England  and    central  New  York.      The 
even  sky  lines  and  other  evidences  of  the  existence  of  this 
peneplain  are  found  in  Pennsylvania  and  far  southward 
along  the  Appalachians.     After  the  Appalachian  revolution, 
vast  denudations  went  on  throughout  the  long  Mesozoic 
periods.     The  land  waste  was  carried  east,  west,  and  south, 
and  a  great  series  of  drainage  modifications  took  place,  into 
which  we  can  not  enter  here,  save  to  remark  that,  in  some 
way  not    yet    clearly  known,  the   northern   Appalachian 
drainage  was  reversed  and  turned  toward  the  Atlantic. 
We  are  chiefly  here  concerned  with  the  destruction  of  the 
mountains  down  to  their  roots,  and  the  formation  of  the 
lowland  surface,  with   its   remnant   mountains  and  hills. 
After  Cretaceous  times  the  peneplain  rose  to  its  present 
position.     Keference  to  this,  however,  will  be  made  in  the 
following  chapter. 

365.  American  Cretaceous  rocks. — A  brief  review  of  these 
will  be  given,  especially  to  show  the  reality  of  the  geo- 
graphic changes  described  in  the  preceding  section.     The 
Cretaceous  system  in   America  is  really  much  more  full, 
both  East  and  West,  than  that  of  the  Jura  Trias.     The 
lowest  Cretaceous  rocks  along  the  Atlantic  border  were  de- 
posited in  fresh  water  and  are  known  as  the  Potomac  for- 


402  GEOLOGY 

mation.  They  consist  of  sandstones,  conglomerates,  and 
clay  beds.  Nothing  is  known  of  the  barriers  which  must 
have  separated  the  waters  that  received  them,  from  the 
open  sea.  Beds  of  corresponding  age  are  known  as  the 
Tuscaloosa  group  in  Alabama  on  the  old  Gulf  border  and 
the  Kootanie  beds  in  the  Rocky  Mountain  region,  Mon- 
tana, Dakota,  and  Canada.  All  point  to  a  time  of  elevation 
and  isolated  belts  of  fresh  water. 

In  the  gulf  that  in  early  Cretaceous  time  stretched 
from  Texas  to  Kansas  a  great  body  of  rocks,  mainly  of 
marine  limestones,  was  laid  down.  They  are  known  as  the 
Comanche  series.  They  include  beds  of  chalk,  like  those 
of  Europe.  The  early  Cretaceous  of  the  Pacific  border, 
west  of  the  new  Sierra  range,  was  also  marine. 

Returning  to  the  East,  we  find  the  upper  Cretaceous 
rocks  with  more  or  less  interruption  exposed  from  Martha's 
Vineyard  through  the  Carolinas.  They  are  found  on 
Block  Island,  Long  Island,  Staten  Island,  and  in  the  States 
of  the  coast  line.  From  central  New  Jersey,  south,  they 
lie  west  of  a  belt  of  overlying  Tertiary  rocks,  which  are  be- 
tween the  Cretaceous  and  the  present  sea  border.  They 
are  chiefly  marine,  and  consist  of  unconsolidated  sands, 
clays,  and  marls.  Overlying  the  Potomac  fresh-water  beds 
as  they  do,  they  prove  the  submergence  noted  in  our  ac- 
count of  the  geographic  changes.  Eipley  group,  Rotten 
limestone,  Eutaw  beds,  and  Tombigbee  sands,  are  names 
given  to  the  later  Cretaceous  beds  of  the  Gulf  border.  In 
the  Rocky  Mountain  region  the  rocks  give  most  interesting 
proof  of  submergence  followed  by  emergence.  Four  epochs 
are  distinguished,  with  corresponding  formations.  They 
are  the  Dakota  (sandstones  and  clays),  Colorado  (lime- 
stones, marls,  shales,  and  sandstones),  Montana  (shales  and 
sandstones),  and  Laramie  (sandstones  and  conglomerates). 
Of  these,  the  first  is  a  fresh-water  formation,  the  second 
and  third  are  marine,  and  the  fourth  is  fresh  water — the 
Coal  period  of  the  West,  as  already  said.  Interested  stu- 


MESOZOIC   ERA 


403 


dents  will  find  accounts  of  these  formations  in  Dana's 
Manual  of  Geology,  and  a  great  body  of  description  in 
the  Government  surveys  of  the  past  thirty  years.  The 
rocks  cover  wide  areas  of  the  "  Great  Plains,"  and  some  of 
them  outcrop  for  long  distances  about  the  base  of  the 
Kocky  Mountains.  Marine  Cretaceous  rocks  were  made  on 
the  Pacific  border,  the  coast  line  running  through  central 
California,  west  of  the  Sierra  Nevada,  and  northward  across 
Oregon  and  Washington. 

366.  Life  of  the  Cretaceous  period.— The  most  remark- 
able progress  is  here  seen  in  the  vegetation.     Fields  and 


FIG.  254.-Leaves  of  Cretaceous  trees-oak,  sassafras,  and  beech. 

forests  begin  to  look  like  modern  times,  because  trees  and 
flowers  appear  like  those  which  are  common  to-day.  This 
is  especially  true  of  the  Laramie  epoch,  which  is  truly  a 
time  of  transition  from  Mesozoic  to  Cenozoic  times.  In 
Europe  there  is  an  important  break  in  the  record  between 
the  two  eras.  This  gap  was  bridged  over  in  an  interesting 
way  by  the  finding  of  the  Laramie  strata.  In  like  manner 
other  obscure  or  unknown  chapters  of  geological  history 
may  become  accessible  through  future  discoveries.  The 
general  principle  of  continuity  in  Nature  is  thus  empha- 
sized. 


404 


GEOLOGY 


Accordingly,  students  of  Laramie  plants  have  inclined  to 
place  the  epoch  at  the  beginning  of  the  Cenozoic  era,  but 
it  has  been  retained  as  the  closing  epoch  of  the  Mesozoic, 

because  of  the  general 
resemblance  of  its  ani- 
mals to  the  older  groups. 
The  change  in  the  plants 
begins  earlier,  however. 
One  fourth  of  the  plant 
species  of  the  fresh-wa- 


FIG.  265.— Branch  and  cone  of  Sequoia. 

ter  Potomac  beds,  lying  at  the  base  of  the  Cretaceous  in 
the  East,  are  Angiosperms  (75  species  out  of  300),  while 
the  Ferns,  Conifers,  and  Cycads  hold  over  from  the  Juras- 
sic period.  Here  also  are  found  Sequoias,  the  great  trees 
of  California,  which  have  also  been  found  in  the  Kootanie 
beds  or  early  Cretaceous  of  British  America,  and  in  Green- 
land. We  have  here  a  single  illustration  out  of  the  multi- 
tudes, to  show  how  the  distribution  of  living  forms  varies 
with  climate,  with  the  extension  and  height  of  the  lands, 
and  all  other  geographical  conditions. 

In  the  later  Cretaceous  the  number  of  existing  forms  is 
greatly  increased,  and  elms,  maples,  beeches,  willows,  sas- 
safras, and  birch  are  common.  Gay  Head  on  Martha's 
Vineyard,  Long  Island,  and  Xew  Jersey  are  some  of  the 
Eastern  localities  where  the  remains  of  these  modern  trees 
have  been  found ;  the  most  important  conclusion  which  is 


MESOZOIC  ERA 


405 


drawn  from  the  character  of  the  Cretaceous  flora  is  that 
warm  climates  were  still  widely  prevalent.      Subtropical 


FIG.  256. — Forest  of  late  Cretaceous  times. 

temperatures  like  those  of  the  Carolinas  or  Cuba  extended 
over  much  of  North  America  and  Europe,  and  far  away  to 
northern  Greenland. 

367.  Animal  life  of  the  Cretaceous  period. — In  some  of 
the  great  groups  the  changes  were  rather  in  details  than  in 
the  chief  patterns,  and  hence  need  not  detain  us.  The 
Protozoa  now  assume  a  recognized  importance  as  rock- 
makers,  since  the  chalks  of  Texas  and  of  Europe  are  largely 
37 


406 


GEOLOGY 


composed  of  their  minute  shells.     Chalks  to  the  thickness 
of  1,000  feet  are  found  in  Europe.     This  fact  bears  testi- 
mony to  the  countless  num- 
bers of  the  organisms,  as  well 
as  to  the  prolonged  time  re- 
quired.    It  is  by  thus  appre- 
ciating   the   time  needed    to 
make  the  rocks  of  a  part  of 
a  single  period  that  the  stu- 


Fio.  857.— Section  of  chalk  rock  much 
magnified,  showing  Protozoan  shells. 


FIG.  258. — Cretaceous  sponge. 


dent  can  at  length  in  an  imperfect  way  realize  the  mean- 
ing of  geological  time  as  a  whole. 

That  sponges  were  abundant  in  some  seas  is  evident, 


FIG.  S59.-Foraminifera  from  chalk  of  Iowa  ( x  100). 


. — Gryphaeti. 


FIG.  260.— Inoceramus. 


••" 


FIG.  264.— A  straight 
Cephalopod  shell, 
Baculite?.  This  is 
like  Orthoceras  in 
general  form,  but 
has  septa  of  Meso- 
zoic  type. 


FIG.  265.— A  high-coiled  Cephalopod  shell,  Turrilitec. 


408  GEOLOGY 

both  from  actual  specimens  and  from  the  vast  quantities 
of  silica  in  the  flint  nodules  of  the  chalk.  Only  occasional 
fossils  of  corals  are  found  either  in  American  or  European 
Cretaceous  rocks.  Great  numbers  of  most  perfectly  pre- 
served sea  urchins  have  been  obtained  from  the  chalk  rocks 
of  Europe,  and  they  form  some  of  the  most  attractive  and 
beautiful  displays  in  the  museums. 

The  Mollusks  were  perhaps  as  abundant  in  the  seas  as 
they  are  now.  A  number  of  large  Lamellibranchs  are  nu- 
merous and  very  characteristic  of  the  Cretaceous.  Such 
are  Inoceramus  and  GryphaBa.  These  are  genera  of  the 
oyster  family.  The  Ammonites  and  Belemnites  continue 
in  great  numbers  from  the  Jurassic,  and  the  Ammonites 
show  interesting  variations  of  form,  which  perhaps  denote 
degeneracy,  for  this  great  Mesozoic  group  disappears  at  the 
end  of  the  Cretaceous  period.  Thus  we  have  Crioceras, 
which  is  an  openly  coiled  shell  (Fig.  262) ;  Hamites,  which 
is  partly  coiled  and  partly  straight;  Turrilites.  which  has 
a  high  coil,  after  the  manner  of  some  Gastropods ;  and  Bac- 
ulites,  which  returns  to  the  straight  form  of  the  Orthoceras, 
but  retains  the  complicated  septa. 

Some  additions  to  the  vertebrate  group  will  be  briefly 
noticed.  Perhaps  chief  among  these  is  the  development 


FIG.  266.— Cretaceous  fish. 


of  bony  fishes  and  the  retirement  of  the  Ganoids  to  a 
minor  place.  During  at  least  four  periods  the  latter  group 
had  been  numerous  and  supreme  among  their  kind.  As 
in  Jurassic  times,  the  reptiles  continued  to  rule  the  liv- 


MESOZOIC  ERA 


409 


ing  inhabitants  of  the  earth,  air,  and  sea. 
The  Mosasaur  is  a  new  form,  of  which 
many  species  have  been  found  in  west- 
ern America,  especially  in  Kansas.  They 
were  extremely  slender,  resembling  both 
the  lizards  and  the  snakes,  and  were 
sometimes  75  feet  long.  They  were  typ- 
ical and  dreadful  sea  serpents.  Snakes 
were  few,  but  lizards  and  crocodiles  were 
common.  Some  flying  reptiles  of  Creta- 
ceous age  had  a  breadth  of  over  20  feet 
when  their  wings  were  spread.  A  great 
addition  to  the  known  bird  fauna  of 
American  Cretaceous  rocks  was  made 
by  the  discoveries  of  the  late  Professor 
Marsh,  of  Yale  University.  Some  had 
teeth  like  the  Jurassic  birds,  but  others 
had  none.  Some  had  but  rudimentary 
wings,  and  accomplished  their  movements 
by  their  long  and  strong  limbs.  They 
varied  in  size,  from  small  birds  to  those 
having  a  height  of  6  feet.  The  mam- 
mals do  not  effect  any  marked  advance 
during  the  period. 

368.  Economic  products  of  the  Creta- 
ceous period. — Much  the  most  important 
of  these  in  America  is  coal.  The  Laramie 
coal  fields  of  the  western  United  States 
are  believed  to  cover  50,000  square  miles. 
To  this  epoch  belong  the  coal  fields  of 
western  Kansas,  New  Mexico,  Colorado, 
Wyoming,  and  Dakota. 

So  far  as  known,  the  Laramie  coal  is 
all  bituminous,  except  in  the  vicinity  of 
the  Elk  Mountains,  west  of  the  Conti- 
nental Divide,  where  subsequent  moun- 


410  GEOLOGY 

tain-building  has  metamorphosed  the  beds  into  anthracite. 
Cretaceous  coals  are  found  also  in  Europe,  as  in  northern 
Germany,  and  other  places  on  the  Continent.  In  Xew  Jer- 
sey the  marls  are  used  as  a  fertilizer,  and  some  of  the  clays 
are  of  a  superior  quality  for  the  making  of  pottery. 


FIG.  268.— Cretaceous  bird,  with  teeth  (half  natural  size). 


CHAPTER  XXIV 

CBNOZOIC  ERA 
TERTIARY  PEBIOD 

369.  WE  now  enter  upon  the  last  of  the  great  eras  of 
geological  history.     It  is,  as  its  name  means,  the  era  of 
new  or  modern  life.     The  higher  vegetation   introduced 
during  the   Cretaceous    period   continues,   and  grows  in 
abundance  and  variety.     The  great  reptiles  have   disap- 
peared and  the  mammals  become  supreme.     From  the  be- 
ginning of  the  era  a  growing  number  of  invertebrate  spe- 
cies are  found,  which  live  on  until  the  present  time.     Geo- 
graphically also,  the  continents  are  approximately  of  their 
present  size  and  form.    Especially  is  the  Tertiary  the  period 
in  which  the  great  mountains  of  the  Old  World — the  Pyre- 
nees, Alps,  and  Himalayas — are  upraised. 

We  divide  the  Cenozoic  era  into  two  periods,  the  Tertiary 
and  the  Quaternary,  or  Pleistocene.  The  name  Tertiary 
survives  from  the  earlier  days  of  geology,  when  the  succes- 
sion of  rocks  with  their  peculiar  characters  was  thought  to 
warrant  a  numerical  classification.  The  names  Primary 
and  Secondary  have  been  discarded,  and  Tertiary  is  re- 
tained, because  it  is  everywhere  used  in  geological  writings, 
and  it  is  easier  to  keep  it  than  to  make  a  change. 

370.  Tertiary  geography  of  North  America. — In  the  east- 
ern part  of  the  United  States,  as  we  have  seen,  the  long 
wear  of  the  Mesozoic  era  had  produced  a  vast  lowland. 
From  New  York  southward  the  early  Tertiary  seas  swept 
in  over  the  eastern  part  of  the  coastal  States,  covering  with 

411 


412  GEOLOGY 

newer  beds  of  gravel,  sand,  and  clay,  most  of  the  Creta- 
ceous deposits.  They  covered  the  region  of  Florida,  and  the 
southern  part  of  Georgia  and  Alabama,  nearly  as  far  north 
as  in  Cretaceous  time,  and  swept  up  about  to  the  mouth  of 
the  Ohio  River.  By  the  close  of  the  period,  however,  the 
eastern  part  of  the  continent  had  risen  higher  than  the 
position  which  it  holds  at  present.  To  this  culminating 
time  of  elevation  belongs  the  elongation  of  the  Hudson  and 
other  Eastern  rivers,  marked  especially  by  the  submarine 
channel  of  the  Hudson,  extending  from  New  York  across 
the  continental  shelf.  The  elevation  also  changed  the  Cre- 
taceous peneplain  into  the  plateau,  now  known  by  the  vari- 
ous names  of  Xew  England  Highlands,  Catskill  Mountains, 
the  Alleghany  Plateau  of  Xew  York  and  Pennsylvania,  and 
the  Cumberland  Plateau,  farther  south.  Gradually  the 
plateau  mass  was  dissected  by  streams  and  weathering  with 
many  modifications  of  drainage,  the  softer  or  more  soluble 
rocks  were  etched  out,  and  the  hard  beds  left  to  show  by 
their  crest  lines  where  the  ancient  peneplain  had  been. 
This,  then,  is  the  period  of  excavation  of  the  Potomac, 
Kanawha,  Susquehanna,  Lehigh,  Delaware,  Hudson,  and 
Connecticut  Valleys.  The  history  of  the  St.  Lawrence  River 
or  of  its  ancient  representative,  for  this  period,  and  of  the 
topography  of  the  Great  Lake  region  during  Tertiary  time, 
still  remains  in  much  obscurity.  It  is  only  when  we  come 
to  the  following  period  that  the  records  become  legible 
and  full. 

We  have  already  seen  that  over  most  of  the  western 
United  States  the  region  of  marine  waters  was  past.  After 
the  Rocky  Mountain  or  Laramie  revolution,  the  region  of 
the  Great  Plains  was  truly  a  part  of  the  continent.  But  it 
is  still  believed  to  have  lain  near  the  sea  level,  with  the 
exception  of  the  mountain  ranges.  The  reason  for  this 
belief  is  found  in  the  existence  in  Tertiary  time  of  vast 
lakes  of  fresh  water,  both  east  and  west  of  the  Rocky 
Mountain  range.  In  some  of  these  lakes  thousands  of 


CENOZOIC  ERA  413 

feet  of  strata  were  formed  from  the  wear  of  the  new-made 
mountains.  It  is  not  believed  that  these  lakes  could  have 
existed  with  the  plains  or  plateaus  at  anything  like  their 
present  altitude.  Some  of  them  were  drained  by  uplift 
with  warping,  as  still  shown  by  the  inclination  of  the 
strata.  The  position  of  the  greater  lakes  will  be  indicated 
in  our  review  of  Tertiary  rocks. 

During  the  Tertiary  period  the  entire  West  rose  by  a 
slow  oscillation  or  series  of  such  movements  by  an  average 
amount  of  several  thousand  feet.  Thus  the  plateau  where 
Denver  lies  is  now  more  than  5,000  feet  above  the  sea. 
Most  of  this  height  was  gained  by  the  gradual  movements 
of  the  Tertiary  period.  Similar  is  the  origin  of  the  present 
altitude  of  the  Great  Basin.  But  there  extensive  faulting 
and  elevation  of  fault-block  mountains  was  going  on.  Sim- 
ilar dislocations  occurred  in  Utah,  forming  its  high  pla- 
teaus ;  also  in  the  Wasatch  Mountains  and  the  Sierra  range. 
The  latter  range  had  been  formed  at  the  close  of  the  Juras- 
sic time,  but  late  in  the  Tertiary  period  it  received  a  great 
part  of  its  present  height  by  elevation  along  the  line  of 
a  profound  fault  on  its  eastern  side,  where  now  is  the 
steep  front  of  the  range.  Taking  into  account  the  wide- 
spread Tertiary  elevation  of  the  Western  country,  Dana 
thinks  that  the  land  mass — that  is,  the  amount  of  matter 
above  sea  level — was  thereby  increased  at  least  tenfold. 

371 .  Volcanic  activity  in  the  Tertiary  period.— This  is  the 
most  extensive  of  which  Xorth  American  rocks  contain 
record,  unless  it  be  in  pre-Paleozoic  times.  The  lavas  in 
various  forms  are  widely  distributed  over  the  mountain  and 
plateau  regions  of  the  entire  Wrest.  Here  belong  the  vast 
lava  sheets  of  the  Columbia  and  Snake  Eivers  in  Idaho, 
Washington,  and  Oregon  ;  the  volcanic  cliffs  and  peaks  of 
the  Yellowstone  Park ;  the  much-denuded  lava  sheets  of 
Colorado,  both  east  and  west  of  the  Eocky  Mountains ;  the 
volcanic  peaks  and  necks  of  Xew  Mexico  ;  the  sheets  of  the 
plateau  region  in  Arizona  and  Utah ;  the  lava  floods  of 


414  GEOLOGY 

the  Great  Basin  ;  and  the  splendid  volcanic  cones  of  Cali- 
fornia, Oregon,  and  Washington ;  Shasta,  Jefferson,  Hood, 
St.  Helen's,  and  Tacoma.  To  the  same  period  are  thought 
to  belong  the  great  volcanoes  of  Mexico  and  the  Andes. 
It  will  be  seen  that  this  period  of  stupendous  outpourings 
was  also  the  time  of  the  great  uplift  of  western  America. 

372.  Epochs  of  the  Tertiary  period.— The  more  common 
division  of  Tertiary  time,  at  least  in  America,  is  threefold, 
giving    us    the   Eocene,   Miocene,   and    Pliocene    epochs. 
Eocene  means  dawn  of  the  recent,  and  is  applied  to  beds 
which  contain  but  a  small  percentage,  5  per  cent  or  less,  of 
invertebrate  species  which  are  now  living.    If  more  of  their 
species  are  living,  up  to  one  half,  the  beds  and  the  epoch  of 
their  making  are  called  Miocene,  which  means  less  recent 
(as  compared  with  the  following  epoch).     If  more  than  50 
per  cent  are  living  species,  the  beds  are  called  Pliocene,  or 
more  recent. 

373.  American  Tertiary  formations  of  marine  origin.— As 
the  western  uplift  of  post-Cretaceous  date  shut  salt  waters 
out  from  the  interior,  the  marine  Tertiary  rocks  are  con- 
fined to  the  Atlantic,  Gulf,  and  Pacific  coasts.     The  only 
known  locality  of  Tertiary  beds  on  the  New  England  coast 
is  on  Martha's  Vineyard.     From  Xew  Jersey  south,  how- 
ever, they  overlie  the  Cretaceous  strata,  and  form  a  nearly 
continuous  belt  of  the  coastal  lowlands,  sloping  gently  off 
beneath  the  sea,  and  widening  from  Xew  Jersey  to  South 
Carolina.    The  Martha's  Vineyard  Tertiary  is  Miocene,  and 
the  Atlantic  coast  Tertiary  chiefly  Eocene  and  Miocene. 
Some  beds  of  New  Jersey  Tertiary  are  composed  of  green- 
sand,  which  consist  largely  of  grains  of  Glauconite  forming 
the  internal  casts  of  minute  Foraminifera,  the  shells  them- 
selves having  been  removed  by  solution. 

The  Gulf  deposits  make  a  wide  belt,  including  all  of 
Florida,  which,  with  Louisiana,  is  the  last  State  to  have 
been  wholly  under  the  sea.  Some  of  the  Gulf  beds  are 
called  Lignitic,  because  they  contain  vegetable  accumula- 


CfiNOZOIC  ERA  415 

tions  made  in  the  swamps  of  the  low-lying  lands.  Buhr- 
stone,  Claiborne,  Jackson,  and  Vicksburg  are  other  names 
of  Gulf  Eocene  deposits.  The  Miocene  epoch  is  repre- 
sented along  the  Atlantic  by  three  stages — the  Chattahoo- 
chee,  Chifola,  and  Chesapeake.  The  beds  of  the  last  are 
exhibited  along  the  shores  of  Chesapeake  Bay  and  of  the 
tidal  rivers  entering  the  bay.  The  Lafayette  is  a  doubtful 
formation  of  unconsolidated,  coarse  sediments  extending 
along  the  South  Atlantic  and  Gulf  coasts,  poor  in  fossil 
contents  and  referred  doubtfully  to  the  Pliocene  and  to 
the  following  glacial  times.  It  should  be  noted  that  the 
majority  of  the  Tertiary  strata  are  unconsolidated,  though 
there  are  many  exceptions,  especially  among  the  calcareous 
deposits. 

We  now  turn  to  the  Tertiary  rocks  of  the  Pacific  coastal 
belt.  The  Coast  Kange  of  the  post-Jurassic  time  is  believed 
not  to  have  been  a  continuous  height  of  land,  but  rather 
an  elongated  archipelago.  Behind  it,  toward  the  Sierra 
Nevada,  was  a  sea,  stretching  far  north  and  south,  where 
the  great  valley  of  California  now  lies.  In  this  sea,  about 
the  islands  and  at  the  western  base  of  the  Sierra,  Tertiary 
muds  were  deposited.  Strata  of  Eocene  age  called  the 
Tejon  Series  are  now  found  at  the  eastern  base  of-  the 
Coast  Eange.  Later  strata  of  Miocene  and  Pliocene  age  are 
found  on  either  side  of  the  great  valley  and  in  the  moun- 
tain masses  of  the  Coast  Eange. 

They  extend  north  into  Oregon  and  Washington.  The 
Tertiary  sea  still  extended  up  the  lower  Columbia  Valley, 
and  was  larger  than  now  in  the  region  of  Puget  Sound. 

374.  Fresh -water  Tertiary  deposits.  —  These  are  quite 
comparable  in  interest  and  importance  to  the  marine  beds. 
The  great  lakes  in  which  these  strata  were  formed  lay 
both  east  and  west  of  the  Rocky  Mountains.*  They  were 

*  Professor  W.  M.  Davis  has  recently  argued  that  some  of  these  so- 
called  lacustrine  deposits  may  have  been  made  by  rivers.  See  the  Fresh- 


416  GEOLOGY 

not  all  contemporaneous,  but  succeeded  one  another  in  the 
several  epochs  of  the  Tertiary  period.  It  will  be  remem- 
bered that  uplifts  and  dislocations  were  taking  place  in  this 
region.  A  lake  might  end  its  existence  because  its  basin 
became  full  of  sediments,  or  because  its  outlet  had  deep- 
ened its  channel,  or  because  the  region  was  tilted  and  the 
lake  waters  gradually  spilled.  The  following  enumeration 
of  the  lakes  follows  Professor  Scott,  who  has  given  pro- 
longed study  to  their  sediments  and  fossil  remains.  The 
Eocene  lakes  fall  under  four  heads  :  (1)  The  Puerco,  in 
northwestern  New  Mexico,  extending  over  into  Colorado ; 
one  of  the  smaller  lakes.  (2)  The  Wasatch  Lakes,  of 
which  the  principal  one  was  of  immense  size,  covering 
much  of  Colorado,  Utah,  and  southwestern  Wyoming. 
Scott  gives  its  length  as  450  miles  and  its  greatest  breadth 
as  250  miles.  (3)  The  Bridger  Lakes,  partly  following  one 
another.  One  of  these  was  north  of  the  Wind  River  Moun- 
tains, and  two  others  lay  along  the  present  line  of  the 
Green  River,  in  Wyoming  and  Utah,  north  and  south  of 
the  Uinta  Mountains.  Another  was  the  Huerfano  Lake, 
south  of  the  Arkansas  River  in  Colorado.  (4)  The  Uinta 
Lake  in  Utah  and  Colorado. 

The  Miocene  lakes  are  as  follows,  as  represented  by 
their  deposits :  (1)  The  John  Day,  covering  a  relatively 
small  area  in  eastern  Oregon.  These  beds  include  vast 
quantities  of  volcanic  ash,  which  fell  upon  the  waters  as 
similar  dust  falls  upon  the  sea  to-day.  (2)  Loup  Fork,  in 
two  stages — the  Deep  River,  whose  waters  spread  over  parts 
of  Montana,  and  the  Nebraska,  whose  beds  stretch  from 
South  Dakota  into  Mexico.  Similar  lakes  existed  in  Cali- 
fornia, Xevada,  and  British  Columbia.  Pliocene  lakes 
were  formed  in  Texas,  Kansas,  Idaho,  and  Oregon.  Ac- 
cording to  Dana,  the  area  of  the  great  Miocene  lakes  now 

water  Tertiary  Formations  of  the  Rocky  Mountain  Region  (Proceed- 
ings of  the  American  Academy  of  Arts  and  Sciences,  March,  1900). 


CENOZOIC  ERA  417 

inclines  from  an  altitude  of  6,000  feet  on  the  west,  near  the 
Kooky  Mountains,  to  3,000  feet  on  the  east.  This  is  due  to 
continental  elevation  with  warping.  These  lake  beds  suf- 
fered vast  erosion,  leaving  valleys,  cliffs,  and  buttes  in  end- 
less variety.  The  climate  is  dry,  the  rocky  wastes  are  un- 
productive, and  hence  the  name  of  "  Bad  Lands  "  is  widely 
used  of  these  regions. 

It  has  already  been  intimated  that  the  great  mountain- 
making  disturbances  of  the  eastern  hemisphere  were  of 
Tertiary  age.  The  Pyrenees  were  raised  at  the  close  of  the 
Eocene,  and  the  Alps  at  the  close  of  the  Miocene.  The 
precise  age  of  the  Himalayas  is  less  well  determined,  except 
that  it  was  after  the  deposits  of  the  Eocene. 

375.  Life  in  the  Tertiary  period. — Angiosperms,  Conifers, 
and  palms  made  up  the  forests  of  the  time  as  they  do  to- 
day.    The  extreme  northern  distribution  of  temperate  and 
subtropical  plants  has  been   noticed  above.     In  harmony 
with  this  a  small  Tertiary  lake  basin  at  Florissant,  Col.,  has 
preserved  the  remains  of  a  warm-temperate  flora  in  a  region 
which  now  is  high  and  too  cool  for  the  raising  even  of 
grain. 

Even  more  do  the  Tertiary  plants  of  Europe  indicate  a 
warm  climate.  They  include  cypress,  magnolia,  figs,  and 
palms,  but  the  temperate  forms  began  to  prevail  before  the 
close  of  the  Miocene  epoch.  From  the  Miocene  of  Oenin- 
gen,  Switzerland,  several  hundred  species  of  plants  have 
been  discovered.  Of  nearly  500  of  these  kinds  Heer  assigns 
more  than  one  half  to  the  subtropical  and  about  one  fifth 
to  the  tropical  classes  of  vegetation. 

376.  Animal  life  of  the  Tertiary  period.— The  life  of  the 
seas  was  steadily  becoming  more  like  that  of  the  present 
day.     As  we  have  seen  in  defining  the  epochs  of  the  period, 
all  its  rocks  contain  more  or  less  of  existing  species,  but  no 
land  species  have  survived  until  now.     This  is  a  great  prin- 
ciple.    Land  species  change  or  become  extinct  sooner  than 
marine,  because  the  environment  also  changes  much  more. 


418 


GEOLOGY 


On  the  lands,  mountain  barriers  are  raised  and  climates 
are  revolutionized,  and  particularly  in  this  case  a  great 
glaciation  came  on,  while  in  the  seas  changes  progress 


FIG.  269.— Tertiary  leaves  and  fruits. 

more  slowly  and  give  time  for  migration.  Xot  much  need 
be  said  as  regards  the  marine  invertebrates.  The  Protozoa 
had  come  to  their  modern  geological  importance.  So  no 
doubt  had  the  Corals,  though  the  rock-making  areas  of  a 


CENOZOIC  ERA 


419 


nearly  completed  continent  were  not  favorable  for  them, 
and  their  more  abundant  relics  are  therefore  beneath  the 


FIG.  270.— Tertiary  Gastropods. 

present  seas.  The  Lamellibranchs  and  Gastropods  looked 
like  those  of  to-day,  and  the  shells  have  suffered  slight 
change  since  first  buried  in  the 
sands  and  clays.  Insects  were 
apparently  as  abundant  as  now. 
They  occur  in  the  lake  beds 
already  referred  to  at  Floris- 
sant, Col.  Some  of  the  shales 
are  like  leaves  of  coarse  paper, 
and  contain  multitudes  of  fossil 
insects  of  all  the  great  types, 
with  large  numbers  of  ants  and 
plant  lice.  Another  famous  lo- 
cality is  in  the  Miocene  of  Oen- 
ingen,  Switzerland,  where  it  is 
reported  that  the  alternation  of 
seasons  can  be  detected  by  the  varying  conditions  of  the 
plants  in  different  layers.  Toward  1,000  species  of  insects 
have  here  been  found,  including  a  great  number  of  wood 


FIG.  271.— A  Tertiary  oyster. 


420  GEOLOGY 

beetles.  Along  the  shore  of  the  Baltic  in  North  Germany, 
also,  2,000  species  of  insects  have  been  found  in  amber,  a 
fossil  gum  from  ancient  coniferous  trees. 

But  it  is  the  vertebrates  which  form  the  great  feature 
of  Tertiary  life.  The  majority  of  the  fishes  are  Teleosts, 
but  some. Ganoids  remain,  and  the  teeth  of  sharks  are  espe- 
cially abundant  in  some  marine  deposits  of  the  southern 
shores.  A  moderate-number  of  snakes,  turtles,  and  croco- 
diles are  found,  but  the  vast  and  characteristic  forms  of  the 
Mesozoic  era  have  all  become  extinct.  The  snakes  have  in- 
terest, because  they  are  degenerate  forms,  having  lost  the 
limbs  which  their  ancestors  possessed. 

377.  Tertiary  mammals. — It  is  these  which  dominate  the 
period,  which  therefore  is  often  called  the  Age  of  Mam- 
mals. In  a  manner  which  geologically  is  sudden,  the  rep- 
tiles waned  and  the  mammals  appear.  The  destruction  of 
the  reptiles  was  not  due  to  the  overmastery  of  the  mam- 


Fio.  272.— Jaw  of  Zeuglodon. 

mals,  but  apparently  to  instability  of  constitution,  or  envi- 
ronment, or  both.  While  the  invertebrate  species  often 
pass  on  into  the  Quaternary  period,  the  mammals  change 
from  epoch  to  epoch,  and  none  of  the  species  survive  Ter- 
tiary time.  The  marine  Tertiary  of  America  does  not 
abound  in  mammalian  remains,  which  are  found  in  mar- 
velous numbers  in  the  lake  deposits  of  the  West.  Primi- 
tive whales,  however,  as  would  be  natural,  occur  in  the  sea 
border  deposits  of  the  Atlantic  and  the  Gulf.  One  of  these 
is  Zeuglodon,  about  70  feet  long,  occurring  in  central  Ala- 
bama. The  name  means  yoke  tooth,  from  the  peculiar 


CENOZOIC  ERA  421 

form  of  the  tooth,  and  the  bones  were  found  in  the  great- 
est abundance,  so  much  so  that  Dana  suggests  an  earth- 
quake shock,  or  some  other  catastrophe,  as  the  cause  of  the 
sudden  death  in  one  place  of  so  many  of  these  creatures. 
The  whale  is  another  example  of  degradation,  being  a  mam- 
mal, descended,  as  is  believed,  from  ancestors  which  lived 
on  the  land.  The  Zeuglodon  just  noted  is  of  Eocene  age. 
Related  species  of  whale  are  found  in  the  Miocene  strata  of 
the  Atlantic  coast. 

The  Creodonts  were  Eocene  creatures,  flesh  eaters,  but 
combined  the  characters  of  true  Carnivores  and  the  insect 
eaters.  As  another  illustration  of  the  law  that  early  forms 
are  apt  to  be  generalized,  true  Carnivores  appear  in  the 
late  Eocene.  Early  Miocene  beds  in  western  America 
show  several  of  them,  including  early  representatives  of 
the  panther,  the  dog,  and  the  cat.  Panthers,  wolves,  and 
tigers  were  plentiful  in  the  late  Miocene  of  the  West.  Eo- 
dents  begin  their  history  in  the  Eocene  epoch,  and,  imme- 
diately following  the  Eocene,  according  to  Scott,  "mar- 
mots, squirrels,  beavers,  mice,  pocket-gophers,  and  rabbits 
are  already  well  established." 
The  reference  is  to  western 
America,  but  it  may  emphasize 
the  antiquity  of  these  groups 
of  familiar  creatures  to  remind 
the  student  that  they  were  thus 
abundant  before  the  Alps  were 
formed. 

The  order  of  Proboscidians 
is  represented   by   elephantine 
creatures    as   far  back   as   the 
Miocene.     The  Tertiary  as  well       FlG  2r3.-Head  of 
as  the  Quaternary  elephants  are 

widely  distributed  in  both  hemispheres.  One  of  these  is 
the  Mastodon,  so  called  from  the  form  of  its  tooth  (nipple 
tooth).  Its  remains  occur  in  Tertiary  strata  of  Texas,  the 


-±22 


GEOLOGY 


Equus : 
Quaternary 
and  Recent. 


Pliohippus : 
Pliocene. 


Protohippus : 
Lower 
Pliocene. 


Miohippus : 
Miocene. 


Mesohippns  : 
Lower 
Miocene. 


Orohippus  : 
Eocene. 


FIG.  274.— Development  of  fore  foot 
(a)  and  hind  foot  (6)  of  the  horse 
during  Tertiary  times. 


Great  Plains,  England,  France, 
Austria,  India,  and  elsewhere. 
Other  elephants  differing  from 
the  Mastodon,  chiefly  in  the 
labyrinthine  patterns  of  their 
teeth,  lived  in  the  Tertiary 
period.  The  Dinotherium  was 
a  huge  creature,  somewhat  like 
an  elephant,  whose  remains  have 
been  found  in  many  parts  of  Eu- 
rope and  in  India.  Its  tusks  ex- 
tended downward,  with  a  back- 
ward curve  (Fig.  273).  Of  other 
familiar  animals,  the  pig,  or  a 
swinelike  creature,  is  found  in 
the  Wasatch  Eocene  beds.  The 
earliest  oxen  were  later,  appear- 
ing in  the  Pliocene  of  the  east- 
ern continents.  The  camels  are 
found  in  Tertiary  times  in  both 
hemispheres,  some  going  back 
as  far  as  the  Eocene. 

The  horses,  tapirs,  and  rhi- 
noceroses have  had  a  long  his- 
tory and  wide  distribution,  go- 
ing back  as  far  as  the  Eocene. 
Especially  important  and  inter- 
esting is  the  ancestral  history  of 
the  horse  as  worked  out  in  de- 
tail by  the  late  Professor  Marsh, 
of  Yale  University,  from  fossils 
found  in  the  Tertiary  lake  de- 
posits of  the  West.  Such  a 
history  affords  most  important 
support  to  the  doctrine  of  evo- 
lution, and  more  and  more  of 


CENOZOIC  ERA 


423 


such  evidence  is  supplied  from  year  to  year  by  the  untiring 
labors  of  the  paleontologists  in  the  study  of  series  of  forms, 
both  invertebrate  and  vertebrate.  The  earliest  known  horse 
is  from  Eocene  deposits,  and  was  about  the  size  of  a  fox. 
It  had  three  toes  on  its  hind  feet,  and  four  and  a  rudiment- 
ary one  on  its  fore  feet.  It  is  called  Eohippus.  The  next 
stage  is  seen  in  Orohippus  from  the  later  Eocene,  in  which 
the  rudimentary  toe  has  disappeared.  Mesohippus  and 
Miohippus  are  Miocene  horses,  which  are  larger,  and  show 
the  fourth  toe  in  a  rudimentary  condition,  as  a  splint,  well 
up  on  the  leg.  In  the  Pliocene  epoch  came  Protohippus 
and  Pliohippus,  with  the  side  toes  shortening  up.  Finally, 
in  Equus,  the  post-Tertiary,  and  existing  genus,  the  hoofs 
of  the  side  toes  have  disappeared  and  are  now  mere  splints. 


FIG.  275. — Mesopithecus,  one  of  the  earliest  Tertiary  monkeys. 

Of  the  order  of  Primates,  the  monkeys  go  back  far  into 
the  Eocene.  In  the  time  of  the  Wasatch,  according  to 
Scott,  they  were  in  great  numbers  and  swarmed  in  the 
trees.  They  appear  also  in  South  American  Tertiary, 


•424 


GEOLOGY 


though  most  of  the  prominent  orders  of  North  American 
mammals  were  not  known  in  the  southern  continent. 
Forms  resembling  the  apes  appear  in  the  Miocene  of 
Europe.  No  human  remains,  whether  bones  or  imple- 
ments, are  of  proved  Tertiary  age. 

378.  Economic  products  of  the  Tertiary  period. — Soft  coals, 
often  in  the  form  of  lignite,  occur  in  many  places,  in  Cali- 
fornia and  Oregon  and  in  Europe,  in  northern  Switzer- 


FIG.  276. — Richmond  earth  under  the  microscope. 

land,  the  Tyrol,  Austria,  and  they  are  of  special  importance 
in  northern  Germany.  The  greensand  of  New  Jersey  is  a 
valuable  fertilizer,  and  likewise  the  phosphates  of  South 
Carolina  and  Florida.  In  Virginia  the  "  Richmond  earth  " 


CENOZOIC  ERA  425 

is  a  deposit,  sometimes  30  feet  thick,  composed  mainly  of 
the  siliceous  shells  of  diatoms.  The  earth  is  used  as  an 
abrasive.  A  similar  deposit  in  Bohemia  was  reported  by 
Ehrenberg  to  contain  many  millions  of  shells  in  a  cubic 
inch.  Kock  salt  and  gypsum  occur  in  the  Tertiary  strata 
of  Europe. 


CHAPTER  XXV 

CBNOZOIC    ERA 

QUATERNARY  OR  PLEISTOCENE  PERIOD 

379.  THIS  period  extends  from  the  close  of  the  Tertiary 
until  the  present  time.     Tertiary  time  closes  and  Quater- 
nary time  begins  with  the  coming  on  of  the  great  glacial 
invasions ;  but  as  these  came  slowly,  we  have  no  narrow 
/ine  of  division.     The  name  Quaternary,  meaning  fourth, 
is,  like  Tertiary,  a  convenient  relic  of  usage.     The  term 
Pleistocene,  often  used,  is  similar  to  the  names  of  the 
Tertiary  epochs,  and  means  most  recent. 

The  best  subdivision  of  Quaternary  time  gives  us  two 
epochs,  the  Glacial  and  the  Post-Glacial  or  Recent.  This, 
again,  is  not  a  sharp  division,  for  the  time  of  final  ice  retreat 
was  not  the  same  for  Illinois  as  for  Labrador,  and  we  may 
say  that  the  Glacial  epoch  is  still  in  progress  in  Greenland. 
Xor  was  the  retreat  necessarily  contemporaneous  on  both 
sides  of  the  Atlantic,  though  the  history  is  believed  to 
have  been  in  fair  correspondence  in  Europe  and  America. 

The  Glacial  Epoch  in  North  America 

380.  The  drift.— This  is  the  name  given  to  rocky  mate- 
rials and  soils  which  have  been  removed  from  their  sources, 
often  to  great  distances.     The  transfer  has  frequently  been 
made  without  regard  to  opposing  slopes  or  the  direction  of 
existing  valleys.     The  term  is  not  used  of  materials  carried 
along  by  rivers,  except  where  the  waters  have  come  from 
melting  glaciers.    The  most  important  element  in  the  drift 

436 


428  GEOLOGY 

is  the  bowlder  clay,  or  till.  The  two  terms  mean  the  same, 
and  refer  to  a  clayey,  sometimes  loamy,  mass  of  unstratified 
material,  which  is  often  filled  with  stones  and  small  or  large 
bowlders,  inclosed  in  it  without  order.  It  often  forms  a 
part  of  terminal  moraines,  but  chiefly  it  makes  up  the 
great  ground  moraine,  or  sheet,  which  covers  the  most  of 
glaciated  •  areas.  It  is  composed  in  part  of  preglacial  soils 
and  stones  loosened  by  weathering  and  pushed  on  by  the 
glacier,  and  in  part  of  rock  flour  and  stones  gathered  from 
the  bed  rocks  by  the  grinding  and  plucking  of  the  glacier 
itself.  These  two  kinds  are  mingled  in  various  pro- 
portions. 

The  mechanically  derived  waste  is  often  detected  by  its 
containing  soluble  minerals,  such  as  lime,  which  would  not 
be  found  in  a  soil  produced  mainly  by  weathering,  or  a 
residual  soil,  as  it  is  called.  This  is  the  difference  between 
the  drift  of  such  a  State  as  New  York  and  the  soils  of  a 
State  like  Tennessee,  over  which  no  glacier  has  passed. 
The  minerals  of  Tennessee  soils  are  such  as  would  be  left 
from  the  local  rocks  after  soluble  matters  have  been  swept 
away.  The  soils  of  southern  New  York,  on  the  other  hand, 
contain  lime  and  other  ingredients  brought  from  central 
and  northern  New  York  and  Canada. 

The  till  is  often  blue  in  color  and  very  tough  and  com- 
pact, having  been  pressed  and  rammed  together  by  the 
weight  and  push  of  the  glacier.  Toward  the  surface  it  is 
often  brownish  or  reddish,  owing  to  post-glacial  leaching 
and  oxidation.  The  overlying  till  is  sometimes  looser  in 
texture,  having  been  less  overridden  or  having  been  dropped 
by  the  ice  in  its  final  melting. 

Another  important  element  in  the  drift  is  its  stratified 
materials.  These  are  sands,  gravels,  and  clays  laid  down 
by  streams  flowing  from  the  glacier  in  glacial  lakes  or  form- 
ing the  kames  and  eskers  already  described  (page  266). 
The  study  of  erratic  bowlders  has  given  us  much  knowl- 
edge of  the  direction  and  character  of  the  movements  of 


CENOZOIC   ERA 


429 


the  ice.  Commonly  the  bowlder  belongs  to  the  till,  but 
may  be  not  infrequently  found  in  the  water-laid  materials. 
The  bowlders,  as  we  have  seen  in  the  chapter  on  Glaciers^ 
may  be  carried  on  the  surface  or  plucked  and  shoved  along 
by  the  under  parts  of  the  glacial  stream.  Some  of  great 
size  have  been  reported.  Several  in  New  England  have 
diameters  of  20  to  60  feet  and  weigh  thousands  of  tons. 


-       . 


FIG.  278.-Drift  bowlder  near  Pittsfleld,  Mass. 

In  central  New  York  are  many  masses  of  stratified  rock, 
10  to  20  feet  across  and  often  10  feet  thick,  which  have 
been  carried  10  to  20  miles  from  known  parent  ledges. 
The  distance  of  transport  greatly  varies,  ranging  from  a 
few  miles,  as  above,  up  to  cases  in  which  a  transport  of  500 
miles  has  been  proved.  Thus  bowlders  are  found  in  Ohio 
which  have  come  from  the  pre-Paleozoic  formations  of 
Canada.  The  bowlders  are  always  of  smaller  average  size 


CENOZOIC  ERA  431 

the  farther  one  goes  from  the  outcrops.  This  is  due  to 
wear  and  splitting  in  transit.  They  are  not  uncommonly 
carried  to  positions  several  hundred  feet  higher  than  the 
beds  from  which  they  came.  Instances  of  an  elevation  of 
several  thousand  feet  have  been  reported  from  some  of  the 
New  England  mountains.  Wherever  a  bowlder  can  be 
traced  to  a  definite  source,  it  determines  the  direction  of 
ice  movement,  at  least  for  one  stage  or  time. 

381.  Striated  rock  surfaces.— These  accompany  the  drift 
and  bear  a  close  relation  to  it,  as  already  explained.     If  on 
the  whole  the  scratches  radiate  from  a  common  ground, 
they  point  to  a  center  of  dispersion  for  the  ice.     They  are 
widely  distributed  over  northeastern  America,  and  tell  the 
story  of  the  general  movements  of  the  ice  sheet  and  of  its 
local  divergences,  as  around  obstacles  and  along  valleys,  in 
front  of  the  ice  or  under  it.    In  general,  the  striae  over  New 
England  point  southeastward,  even  disregarding  the  ridges 
of  the  Green  Mountains.     Over  New  York  the  general 
direction  is  southward,  and  farther  to  the  west  the  com- 
mon   trend  is    to   the   southwest.     Considering  these  as 
radial    lines   and  tracing    them   back,    they  intersect    in 
British  America.    The  direction  of  the  bowlder  movements 
offers  similar  evidence,  and  leads  us  to  sure  conclusions  as 
to  the  general  course  of  the  ice. 

382.  Abandoned  theories  of  the  drift. — During  most  of  the 
first  half  of  the  century  many  of  the  above  facts  were  seen, 
and  honest  but  curious  efforts  made  to  explain  them  by 
theories   of   floods   and  icebergs.      The  confused  stratifi- 
cation of  glacial  sands  pointed,  as  was  thought,  to  the 
tumult  of  such  waters,  and  the  southerly  direction  of  car- 
riage was  held  to  prove  that  vast  currents  of  water  had 
come  from  the  north.      The  scratches  were  explained  as 
the  work    of    icebergs    grazing   the    bottom.     President 
Hitchcock  is  an  example  of  a  powerful  mind  and  keen 
observer  grappling  with  the    facts  with  the  inadequate 
theories  of  his  time.      The  weak  points  of  the  Diluvial 


432 


GEOLOGY 


Theory  were  that  it  did  not  explain  the  unstratified  drift, 
the  continuity  of  the  scratches,  and,  above  all,  that  the 
source  of  the  waters  was  purely  imaginary. 

383.  The  glacial  origin  of  the  drift. — This  was  proposed 
by  Louis  Agassiz,  about  1840,  from  his  study  of  glacial  phe- 


v  .*- 


FIG.  280.— Recently  glaciated  cliff  over  the  Aar  glacier.  The  newly  rubbed  surface 
is  about  60  feet  high  and  shows  in  the  lighter  shade.  The  glacier  ice  in  the  fore- 
ground is  free  from  morainic  material  and  looks  white,  like  snow,  because  of  sur- 
face melting.  Photograph  by  the  author. 

nomena  in  Switzerland.  Far  above  the  glaciers  and  far 
down  the  valleys  are  found  striated  rocks  and  morainic 
masses.  Erratic  blocks  are  common,  and  have  even  been 
carried  across  the  great  valley  of  Switzerland  and  left  on 
the  sides  of  the  Jura  Mountains.  Gradually  the  theory  was 


CENOZOIC  ERA 


433 


accepted  in  Great  Britain  and  America,  and  it  was  recog- 
nized that  wide  fields  of  ice,  far  greater  than  those  of  Green- 
land at  the  present  time,  had  covered  the  most  of  northern 
Europe  and  America.  Thus  the  essential  facts  of  the  drift, 
of  striated  rock  surfaces,  and  of  the  topography,  are  ex- 


FIG.  281.— Glaciated  spur  in  the  Aar  Valley.    Photograph  by  the  author. 


plained.  It  is  at  the  same  time  plain  that  water  had  much 
to  do  with  shaping  the  drift  along  the  lines  of  the  ice 
front,  in  valleys,  and  in  low  grounds  by  the  sea.  Icebergs 
there  were,  and  sometimes  in  abundance,  but  the  great  work 
was  done  by  land  ice. 

384.  The  centers  of  dispersion.— In  the  light  of  present 
studies  there  seem  to  have  been  three  important  centers  in 


434  GEOLOGY 

North  America.  The  first  of  these,  long  recognized,  lay  be- 
tween the  St.  Lawrence  Eiver  and  Hudson  Bay,  forming  the 
highlands  of  Canada.  Thence  the  ice  extended  over  much 
of  Canada  and  the  northeastern  United  States  (Laurentide 
Ice-sheet).  Another  gathering  ground  of  ice  is  believed  to 
have  been,  on  the  low  grounds  about  Hudson  Bay  westward, 
with  movements  far  southward,  over  the  plains  of  British 
America  and  the  United  States,  and  nearly  to  the  base 
of  the  Canadian  Kocky  Mountains  (Keewatin  Ice-sheet). 
Another  extensive  glacial  sheet  occupied  that  mountain 
range,  perhaps  reaching  over  to  the  great  sheet  on  the 
plains  to  the  eastward,  and  sending  out  tongues  to  the 
Pacific  on  the  west,  as  the  Greenland  glaciers  now  do  into 
the  surrounding  seas.  Farther  south  in  the  United  States 
powerful  local  glaciers  formed  in  the  mountains  both  of 
the  Continental  Divide  or  Rocky  Range  and  in  the  Sierra 
Nevada  and  its  northern  extensions  in  Oregon  and  Wash- 
ington. The  glaciers  of  Mount  Rainier,  Mount  Shasta,  and 
other  peaks  are  shrunken  remnants  of  the  greater  streams. 
In  the  Rocky  Range  they  were  found  well  south  in  Colo- 
rado. A  glacier  60  miles  long  extended  down  the  Animas 
River  Valley  in  southwestern  Colorado.  Colorado  Springs 
is  now  supplied  with  water  from  Lake  Moraine,  a  small 
lake  held  in  by  a  morainic  dam  halfway  to  the  summit  of 
Pike's  Peak.  The  glaciers  did  not  there  come  down  to  the 
plains.  It  should  be  observed  that  the  ice  flowed  northward 
from  the  gathering  grounds  in  British  America,  but  failed 
to  reach  much  of  the  territory  bordering  the  arctic  seas. 

A  word  should  be  added  as  to  the  manner  of  dispersion. 
It  is  not  to  be  supposed  that  all  the  ice  which  covered  New 
York,  for  example,  came  from  the  Canadian  highlands. 
No  doubt  much  ice  was  formed  by  the  Adirondack  snows, 
a  region  which  may  thus  be  called  a  local  center.  The 
same  is  probably  true  of  the  New  England  mountains  and 
other  high  grounds.  That  the  great  northern  centers  were 
important  and  dominated  the  whole  territory  is  still  evident 


CENOZOIC  BRA 


435 


from  the  course  of  the  scratches  and  the  distribution  of  the 
far-traveled  erratics.  How  the  warm  climate  of  middle 
Tertiary  became  so  changed  is  a  question  which  is  deferred 
to  a  later  section  in  this  chapter.  No  doubt  snows  fell 
widely  over  the  broad  areas  affected,  with  many  places  of 


G.  282.— Glaciated  conglomerate,  near  Ouray,  Col.    Photograph  by  the  author. 


abundant  precipitation,  especially  those  mentioned  in  the 
north.  It  is  not  easy  to  think  adequately  of  the  long  time 
which  must  have  been  required  for  the  ice  to  creep  over  an 
area  of  some  millions  of  square  miles. 

385.  The  glacial  boundary  in  the  United  States.— In  trac- 
ing this  line  the  student  should  be  reminded  that  the  ice 


436  GEOLOGY 

did  not  occupy  the  whole  of  this  outermost  line  of  advance 
at  one  time.  It  seems  to  have  melted  far  back  from  the 
western  part  of  the  line  long  before  it  left  the  eastern  por- 
tion. All  of  New  England  was  covered  by  ice,  and  the 
front  was  in  the  sea,  or  perhaps  along  the  line  of  the  south- 
ern islands,  since  belts  of  moraine  cross  Nantucket,  Martha's 
Vineyard,  Block  Island,  and  Long  Island.  Professor  Shaler 
has  found  a  fan-shaped  train  of  bowlders  of  a  peculiar  ore 
of  iron  leading  south  from  northern  Khode  Island,  one 
erratic  piece  appearing  on  Martha's  Vineyard.  From  Long 
Island  and  Staten  Island  the  low  morainic  belt  crosses  cen- 
tral New  Jersey.  Thence  it  runs  northwesterly  into  south- 
western New  York,  where  a  small  corner  of  that  State  re- 
mained free  from  the  ice  cover.  Thence  the  student  may 
follow  its  general  direction  on  the  map  southwest  to  Cin- 
cinnati, curving  across  southern  Indiana  and  Illinois  west- 
ward, nearly  along  the  line  of  Missouri  Eiver  into  Kansas. 
Thence  the  line  stretched  northwestward  across  Nebraska, 
South  and  North  Dakota,  and  through  Montana,  where  the 
territory  of  the  northern  Cordilleran  glacial  field  is  entered. 
The  student  must  not  suppose  that  this  margin  is  all  the 
way  marked  by  a  belt  of  moraines.  Particularly  along  the 
broad  stretches  of  the  Mississippi  Valley  moraines  are  ab- 
sent, and  we  have  what  Chamberlin  has  called  an  "  attenu- 
ated pebbly  border,"  showing  drift  but  no  topographic 
feature.  This  is  quite  natural.  The  ice  front  may  have 
been  temporary  in  that  southern  region,  and  accumulations 
then  made  were  likely  to  be  swept  away  and  redistributed 
by  the  great  discharge  of  waters  from  the  melting  ice  which 
long  sought  that  way  to  the  sea. 

386.  Thickness  of  the  ice. — This  varied  with  the  surface 
over  which  it  moved.  Thicknesses  of  a  mile  or  more  were 
probably  common,  with  smaller  measures  toward  the  margin 
and  perhaps  much  greater  in  the  centers  of  accumulation. 
The  Adirondack  and  White  Mountains  seem  to  have  been 
covered.  This  accords  with  the  above  figure.  The  top  of 


FIG.  283. 


438  GEOLOGY 

Mount  Katahdin  is  thought  to  have  been  a  nunatak,  or 
island  rising  through  the  ice.  The  flow  of  a  sheet  rela- 
tively so  thin,  for  long  distances  over  rough  ground  seems 
surprising.  But  an  inclination  of  the  ice  surface  of  40  feet 
per  mile  is  sufficient  for  flow  in  Greenland  to-day,  and  we 
must  also  remember  that  this  portion  of  the  continent  was 
much  higher  than  now — how  much  is  not  well  known. 

387.  Subdivisions  of  the  Glacial  epoch. — Final  statements 
can  not  be  made,  but  students  of  glacial  phenomena  in  the 
great  Mississippi  region  are  agreed  that  there  were  several 
important  advances  and  retreats  of  the  ice.  The  limits  of 
the  more  important  advances  are  well  known,  but  the  dis- 
tances to  which  the  ice  retired  to  the  north  in  these  warm 
inter-glacial  intervals  are  not  so  well  determined.  That  the 
retreats  were  much  more  than  fluctuations  of  the  margin  is 
believed  for  several  reasons.  Beds  of  vegetable  soil,  with 
plant  remains,  are  found  between  younger  and  older  sheets  of 
till.  These  remains  indicate  a  warmer  climate  than  would 
be  found  close  to  an  ice  sheet.  The  outer  and  older  sheets  of 
till  are  much  more  weathered  and  denuded  than  the  younger, 
thus  proving  difference  in  time.  The  inter-glacial  inter- 
vals were  times  of  submergence  or  low-lying  lands,  as 
well  as  of  warmer  temperatures.  Among  the  more  impor- 
tant sub-epochs  or  periods  of  advance,  we  note  the  Kansan, 
marking  the  farthest  extension  of  the  ice  into  eastern  Kan- 
sas; the  lowan,  marked  by  a  sheet  of  till  reaching  into 
northeastern  Iowa ;  and  the  Wisconsin,  marked  by  a  splen- 
did series  of  deposits  in  that  State.  The  last  were  first  stud- 
ied by  Chamberlin,  and  named  by  him  the  Kettle  Moraine. 
Other  and  intermediate  stages  have  also  been  made  out. 
The  Wisconsin  stage  is  thought  to  be  contemporary  with 
the  farthest  extension  of  ice  in  Pennsylvania  and  eastward. 
Thus,  if  there  were  retreats  and  advances  as  extensive  in 
the  East  as  in  the  West,  the  deposits  were  overridden  and 
the  evidence  obscured.  Dana  thinks  that  greater  supplies  of 
snow  in  the  East  kept  up  the  flow,  and  pushed  the  glacial 


CENOZOIC  ERA 


439 


limit  ever  well  to  the  southward.  But  some  evidence  for 
the  diversity  of  the  Glacial  epoch  in  the  East  has  been 
found. 

388.  Advance  of  the  ice  as  affected  by  topography.— An 
advancing  sheet  would  flow  about  an  opposing  hill  or  moun- 
tain, and  if  the  supply  was  great  enough  would  overtop  it 
and  cover  it  from  sight.     Valleys  lying  more  or  less  in  the 
direction  of  flow,  would  be  entered  first  by  tongues  project- 
ing forward  perhaps  a  number  of  miles  from  the  main 
body.     Great  valleys,  such  as  are  occupied  by  the  Great 
Lakes,  were  filled  by  vast  ice  lobes.     Such  lobes  occupied 
the  basin  of  Lake  Superior,  Lake   Michigan,  Lake  Erie, 
and  others.     Gradually  the  ice  covered  the  intervening 
higher  grounds  and  moved  out  upon  the  central  Mississippi 
Valley. 

389.  Recession  of  the  ice. — Why  the  climate  changed  is  a 
question  for  later  consideration.     We  are  concerned  now 
only  with  the  fact  and  its  consequences.    The  sheet  did  not 
melt  away  at  a  uniform  rate,  but  with  long  pauses  from 
time  to  time,  in  which  moraines  were  accumulated.     Even 
in  these  pauses  the  front  was  not  stationary,  but  fluctuated 
like  those  of  the  Swiss  glaciers.     Hence  the  moraines  are 
not  single  ridges,  but  belts  of  rough  ground.     Many  belts 
of  moraine  may  be  traced  in  Minnesota,  Iowa,  and  west- 
ward.    Especially   do   we   see   them  as  successive    belts, 
south  of  the  lakes  in  Illinois,  Indiana,  Ohio,  and  Michi- 
gan, marking  the  shrinking  of  the  great  ice  sheet.     Such 
belts  form  the  terminal  moraine  south  of  Xew  England, 
and  are  seen  across  New  York  and  Massachusetts  and 
elsewhere. 

390.  Floods  from  the  melting  ice  sheet.— The  ice  in  its  full- 
est extension  lay  far  over  in  the  basins  of  the  Susquehanna, 
Ohio,  and  Mississippi  Rivers.    As  melting  proceeded,  these 
great  rivers,  by  means  of  their  branches,  gathered  the  dram- 
age  and  carried  the  waters  to  the  sea.     The  coarser  waste, 
and  in  times  of  low  altitude  of  the  lands,  fine  clays  as  well, 


CENOZOIC  ERA  444 

were  strewn  along  the  valleys.  These  are  now  found  as 
valley  trains  of  aqueo-glacial  debris,  filling  the  valleys  of 
the  Northern  States,  often  to  depths  of  several  hundred 
feet.  At  intervals  in  these  valleys  kames  are  often  found, 
which  are  moraines,  formed  during  a  pause  in  the  recession. 
In  front  of  them  overwash  aprons  or  deltalike  terraces 
often  appear,  and  glacial  lakes  and  swamps  are  common. 
The  valleys  of  the  central  New  York  plateau  offer  many 
examples. 

391.  Glacial  lakes,  north  of  the  Mississippi-Hudson  Bay 
and  Mississippi-St.  Lawrence  watersheds. — As  the  ice  front 
began  to  lie  to  the  north  of  these  watersheds,  lakes  formed 
between  their  height  of  land  and  the  ice  lying  to  the  north 
of  it.  Their  outlets  passed  through  the  lowest  notches  in 
the  divide,  and  carried  the  waters  to  the  Gulf  of  Mexico 
and  the  Chesapeake  Bay.  One  of  these  old  outlets  is  seen  in 
Brown's  Valley  in  western  Minnesota,  where  a  typical  stream 
bed  crosses  the  watershed,  but  is  now  dry,  save  for  two  shal- 
low lakes.  The  lake  that  formed  to  the  northward  gradu- 
ally became  longer  as  the  ice  melted  back,  until  it  extended 
over  700  miles  through  western  Minnesota,  eastern  Dakota, 
and  far  into  Manitoba,  covering  the  area  of  the  present 
Winnipeg  Lake.  This  lake  is  called  Lake  Agassiz,  in  honor 
of  the  author  of  the  Glacial  Theory.  The  successive  beaches 
of  this  lake  have  been  traced  for  hundreds  of  miles  on  the 
east  and  west  sides  of  the  area.  Most  geologists  believe 
that  the  retreating  ice  front  was  the  barrier  which  held  it 
on  the  north.  An  elaborate  report  on  Lake  Agassiz,  by  Mr. 
Warren  Upham,  is  published  by  the  United  States  Geologi- 
cal Survey.  The  muds  deposited  in  it  form  great  areas  of 
valuable  wheat  lands.  Similarly  lakes  were  formed  in  the 
region  of  the  head  of  Lakes  Superior,  Michigan,  and  Erie,  as 
the  ice  gave  way.  One  great  avenue  of  discharge  was  near 
the  site  of  Chicago,  by  the  valley  of  the  Illinois  Eiver  to  the 
Mississippi.  Another  was  across  the  watershed  of  Fort 
Wayne,  Ind.  Yet  another  was  through  the  valley  of  Sene- 


4:42 


GEOLOGY 


ca  Lake  over  the  site  of  Horseheads  and  Elmira,  N.  Y.  As 
the  ice  receded  and  the  lakes  grew,  they  merged  into  one 
another  at  lower  and  lower  levels,  forming  vast  sheets  of 
water.  One  of  these  is  called  Lake  Warren.  Its  beaches 
now  lie  about  870  feet  above  sea  level,  and  have  been  traced 


FIG.  285.— Scratched  stone  from  the  bowlder  clay  of  central  New  York. 


from  western  New  York  far  westward.  The  outlet  was  to 
the  west,  across  the  site  of  Chicago  and  along  the  line  of 
the  Drainage  Canal.  These  lakes,  as  most  believe,  were 
held  by  glacial  dams  on  the  north  and  east. 

The  time  came  when  the  ice  began  to  melt  out  of  the 
Mohawk  Valley  in  eastern  New  York,  and  thus  to  offer  a 
lower  path  to  the  sea  than  could  be  had  by  the  Horseheads, 
Fort  Wayne,  or  even  by  the  Chicago  region.  Most  remark- 
able abandoned  channels  of  these  eastward  streams  cross 
the  high  ridges  of  land,  a  few  miles  south  of  Syracuse, 


CENOZOIC  ERA  443 

N.  Y.  Rocky  beds,  terraces,  deltas,  and  cliffs,  whose  water- 
falls rivaled  Niagara,  are  there  seen  in  wonderful  perfec- 
tion. The  present  divide  between  the  Mohawk  and  Lake 
Ontario  basins  at  Rome,  X.  Y.,  is  445  feet  above  the  sea. 
When  this  depression  was  clear,  all  the  drainage  of  the 
Great  Lakes  sought  the  Atlantic  by  way  of  the  Mohawk 
and  Hudson  Valleys.  When  the  vast  expanse  of  waters  was 
thus  drawn  down,  the  Niagara  escarpment  was  uncovered 
in  western  New  York  and  Ontario.  Lake  Erie  was  sepa- 
rated from  its  northern  neighbor,  the  drainage  of  Erie  and 
the  upper  lakes  found  its  way  across  the  plateau  north  of 
Buffalo  to  the  edge  of  the  escarpment,  and  the  Niagara 
Falls  began.  Niagara  is  a  mammoth  example  of  what  hap- 
pened in  a  multitude  of  instances  on  the  retirement  of  the 
ice.  The  waters,  not  able  to  find  their  original  lower  chan- 
nel, gained  a  higher  outlet  across  the  plateau.  This  plateau 
being  bounded  on  the  north  by  a  cliff,  a  great  waterfall 
came  into  being.  Niagara,  with  its  gorge  and  rapids,  has  an 
elaborate  history  which  can  not  be  given  here.  Interested 
students  should  consult  the  writings  of  Gilbert,  Taylor, 
Spencer,  and  others. 

The  greater  ancestor  of  Lake  Ontario,  which  then  had 
its  outlet  at  Rome,  is  called  Lake  Iroquois,  from  the  early 
Indian  confederacy  of  New  York.  Its  ancient  beaches  ex- 
tend through  western  New  York  to  the  opening  of  the  Mo- 
hawk Valley  and  northward  to  Watertown.  At  length  the 
glacier  melted  out  of  the  St.  Lawrence  Valley,  the  Rome 
outlet  was  abandoned,  and  the  present  drainage  established. 
The  reality  of  these  ancient  bodies  of  water  is  vividly  proved 
by  the  elevated  beaches  which  are  conspicuous  about  all  the 
Great  Lakes.  Around  Lake  Superior  they  are  found  at  in- 
tervals to  a  height  of  more  than  500  feet  above  the  lake. 
In  Ohio  and  Michigan  they  run  about  the  lake,  roughly 
parallel  to  the  present  shores.  Others  are  seen  about  Lakes 
Michigan  and  Huron.  They  are  not  now  horizontal,  hav- 
ing been  carried  up  on  the  northeast  by  warping  of  the  con- 


444  GEOLOGY 

tinent.  Thus  the  old  beach  of  Lake  Iroquois  is  116  feet 
above  Lake  Ontario,  at  the  west  end,  but  485  feet  above  at 
Watertown  on  the  northeast. 

392.  Lake  Bonneville. — A  great  body  of  water  of  which 
Great  Salt  Lake  is  the  shrunken  representative,  formerly 
occupied,  much  of  Utah  in  the  eastern  part  of  the  great 
basin.     It  is  called  Bonneville,  after  the  explorer,  Captain 
Bonneville,  who  visited  the  region  about  1833.     The  exist- 
ence of  a  larger  lake  is  proved  by  a  series  of  very  perfect 
beaches,  on  the  adjacent  mountain  slopes.    A  brief  account 
of  this  lake  is  found  on  page  279.     Eeference  is  here  again 
made  to  it,  because  it  probably  belongs  to  the  Glacial  epoch, 
when  precipitation  was  much  greater  in  that  region,  and 
great  glaciers  occupied  the  valleys  of  the  Wasatch  Moun- 
tains on  the  east.     A  similar  lake,  smaller  and  of  less  com- 
pact form,  occupied  the  low  grounds  of  Nevada,  and  is 
known  as  Lake  Lahontan. 

393.  The  Champlain  depression. — Eeference  has  already 
been  made  to  the  inter-glacial  intervals  characterized  by 
low-lying  lands,  warm  climate,  and  sluggish  stream  action. 
These  are  most  clearly  shown  by  the  succession  of  deposits 
in  the  West. 

It  is  also  known  that  a  great  sinking  of  the  lands 
took  place  during  the  waning  of  the  ice  sheet,  and  after 
its  departure  in  the  East.  This  stage  has  long  been 
called  Champlain,  from  marine  deposits  on  the  borders 
of  Lake  Champlain.  The  depression  was  enough  to  al- 
low the  sea  to  extend  over  Manhattan  Island,  up  the 
Hudson  Valley.  Marine  waters  in  the  Champlain  Val- 
ley merged  with  a  great  St.  Lawrence  Gulf  and  ex- 
tended into  Lake  Ontario.  The  records  of  the  subsi- 
dence are  found  first  in  a  series  of  terraces,  extending 
along  the  Hudson  and  Champlain  Valleys.  They  consist 
of  sands,  gravels,  and  clays,  and  are  conspicuous  features 
of  the  landscape.  At  the  mouths  of  side  valleys,  like 
the  Fishkill,  Catskill,  Mohawk,  and  Hoosick,  they  take 


CENOZOIC  ERA  445 

the  form  of  deltas.  These  terraces  mark  a  submergence 
of  70  feet  at  New  York,  180  feet  about  Xewburg,  over 
300  feet  at  Albany,  and  from  400  to  500  feet  on  Lake 
Champlain.  In  the  Champlain  terrace  of  the  Vermont 
side  the  remains  of  a  whale  were  found.  The  terraces  are 
still  higher  on  the  St.  Lawrence.  Kaised  beaches  show  a 
submergence  of  200  to  300  feet  on  the  coast  of  Maine. 
Stratified  glacial  deposits  more  than  4,000  feet  thick  are 
found  on  the  coast  of  Alaska,  indicating  subsidence  and 
later  uplift  to  that  amount  in  that  region.  This  may 
perhaps  have  been  contemporary  with  the  subsidence  in 
the  East. 

394.  Kiver  drainage  re-established. — As  the  ice  retreated, 
the  surface  waters  resumed  their  work  with  somewhat 
changed  conditions.  The  preglacial  valleys  had  been  very 
generally  graded  up  with  waste,  either  coarse  materials  de- 
posited by  swift  streams  from  the  glacier,  or  clays  deposited 
by  sluggish  streams  and  in  lakes,  during  times  of  depres- 
sion. These  buried  or  half-filled  valleys  are  known  chiefly 
through  borings  made  for  oil,  gas,  water,  or  other  products. 
All  the  streams  entering  Lake  Erie  from  the  south  flow 
far  above  the  ancient  valley  bottoms.  From  200  to  more 
than  300  feet  of  fine  clays  lie  in  some  valleys  of  central 
New  York. 

In  many  cases  a  valley,  or  a  section  of  it,  was  blockaded 
by.  morainic  waste,  and  the  streams  were  unable  there  to 
maintain  their  ancient  courses.  As  a  result,  lakes  were 
formed  above  the  barrier,  and  a  gorge  cut  past  the  block- 
ade at  a  greater  or  less  distance  from  it,  until  the  lake  was 
drained.  Illustrations  of  such  changes  have  already  been 
given.  Sometimes  a  buried  channel  is  tapped  with  disas- 
trous results  in  mining.  In  1885  the  roof  of  a  coal  mine 
near  Xanticoke,  Pa.,  caved  in,  and  a  flood  of  glacial  gravel 
entrapped  a  number  of  miners. 

In  some  cases  important  diversions  or  reversals  of  drain- 
age occurred.  The  head  waters  of  the  Alleghany  are  very 


446  GEOLOGY 

close  to  Lake  Erie,  but  borings  have  proved  that  the  rock 
floors  of  valleys  of  northwestern  Pennsylvania  descend 
toward  Lake  Erie.  Glacial  materials  have  graded  them  up 
and  turned  the  surface  slope  to  the  south.  In  a  similar 


FIG.  886.— Iroquois  shore  near  Pierrepont  Manor,  N.  Y.  Terrace  cut  from  till,  and 
set  with  bowlders  derived  from  it.  G.  S.  A.  Photographs  No.  188.  Photograph 
by  G.  K.  GILBERT. 

manner  the  head  waters  of  the  Mohawk  were  diverted  from 
their  preglacial  connection  with  the  St.  Lawrence  system. 

395.  Topographic  products  of  glaciation.— Some  of  these 
have  already  been  noticed.  Such  are  kettle-hole  lake  basins, 
Morainic-barrier  basins,  kames,  drumlins,  and  eskers.  In 
eastern  Massachusetts  and  elsewhere  occur  sand  plains,  or 
fossil  deltas,  made  in  temporary  bodies  of  water  at  the 
ice  front,  and  since  exposed  and  sometimes  dissected. 
Great  plains  of  washed  gravel  and  sand  stretch  away  from 
ancient  ice  limits,  where  the  ice  deployed  on  level  grounds, 


CENOZOIC  ERA  447 

while  in  a  hilly  region  the  washed  material  forms  trains 
in  the  valleys.  In  cases  of  sufficient  uplift  and  vigorous 
flow  of  water  since  the  time  of  valley  filling,  the  valley 
trains  may  be  partly  cut  away  and  a  system  of  terraces 
formed.  While  low  grounds  were  in  many  cases  graded 
up,  it  is  also  true  that  higher  and  exposed  grounds  and 
isolated  rock  masses  suffered  abrasion  and  often  consider- 
able degradation.  Facts  are  not  available  to  show  how  much 
the  tops  of  mountains  and  hills  have  suffered  in  this  way  ; 
but  we  know  that  ordinary  subaerial  denudation  produces 
narrow  hilltops  and  sharp  spurs  in  a  region  of  consider- 
able topographical  relief.  We  can  not  doubt  that  many 
such  areas  existed  before  the  ice  invasion,  where  now  we 
find  subdued  forms,  rounded  summits,  and  low  drumlinoid 
hills.  The  general  tendency  has  been,  therefore,  to  fill  up 
depressions,  pare  down  elevations,  and  reduce  the  surface 
to  uniformity.  On  the  other  hand,  such  topography  is  re- 
lieved by  morainic  hills  and  by  post-glacial  gorges,  and  it 
is  possible  that  natural  scenery  as  a  whole  is  more  varied 
because  of  the  great  invasion. 

396.  The  driftless  area  of  the  upper  Mississippi  Valley. — 
The  above-named  principles  find  illustration  in  a  large 
tract  lying  mainly  in  Wisconsin,  over  which  glacial  ice  did 
not  move.  It  has  an  area  of  about  10,000  square  miles,  and 
extends  from  central  Wisconsin,  a  little  past  the  south  and 
west  boundaries,  into  Illinois  and  Iowa.  It  has  great  value 
as  a  standard  of  comparison  with  glaciated  areas,  lying  in 
the  same  latitude  and  having  similar  rock  formations.  On 
the  north  and  east  the  area  is  bordered  by  heavy  moraines, 
marking  the  bounds  of  the  Lake  Superior  and  Lake  Michi- 
gan ice  lobes.  There  are  no  moraines  on  the  south  and 
west  borders.  The  soils  are  residual — that  is,  formed  from 
the  underlying  rocks,  and  poor  in  the  soluble  elements, 
such  as  lime.  The  transition,  as  in  the  southern  unglaci- 
ated  regions,  is  gradual,  from  the  soil,  to  decayed  and  then 
to  the  unmodified  bed  rock.  The  average  thickness  of  the 


448 


GEOLOGY 


soils,  taking  1,800  trials  into  account,  is  about  7  feet. 
There  is  no  erratic  material  except  that  brought  in  from 
the  glaciated  border,  along  the  valleys. 

The  topography  differs  much  from  that  of  the  surround- 
ing territory.  There  are  no  waterfalls,  while  they  are  common 
on  the  borders.  The  valleys  are  wide,  with  flowing  slopes 
or  with  recession  cliffs,  but  there  are  no  narrow  gorges. 
There  are  frail  remnant  pillars  of  rock  within  the  region, 
but  none  outside.  The  drainage  system  is  perfected,  and 
there  are  no  lakes.  Valley  trains,  with  drift  material,  cross 


Fio.  287.— Glacial  flutings  of  bed  rock,  near  Burlington,  la. 

the  tract,  as  along  the  Wisconsin  River,  which  comes  in  from 
the  glaciated  area  on  the  northeast.  The  cause  of  such  a 
gap  in  the  glaciated  territory  is  not  well  understood.  One 
suggested  reason  is  the  control  of  the  great  ice  currents  by 
the  Lake  Superior  and  Lake  Michigan  valleys,  carrying  the 
ice  on  either  side  of  the  higher  grounds  of  northern  Wis- 
consin.* Interested  students  may  find  a  full  account  in  a 

*  A  later  theory  is  that  the  Laurentide  glacier  failed  to  reach  the  re- 
gion from  the  east,  while  the  Keewatin  glacier  fell  short  of  it  on  the  west. 


CBNOZOIC  ERA  449 

paper  by  Chamberlin  and  Salisbury  in  the  Sixth  Annual 
Report  of  the  United  States  Geological  Survey. 

397.  The  Glacial  epoch  in  other  lands.— The  facts  about 
the  ice  invasion  in  Europe  are  well  known.  From  the  moun- 
tains of  Scandinavia  the  ice  appears  to  have  moved  in  every 
direction,  though  the  limits  are  naturally  less  known  on  the 
north  than  elsewhere.  Great  Britain  and  Ireland  were 
nearly  covered,  the  ice  coming  down  almost  to  the  line  of 
the  Thames.  As  in  America,  the  Glacial  epoch  was  largely 
a  time  of  elevation.  The  bottoms  of  the  shallow  Xorth  Sea 
were  land,  and  occupied  by  ice,  which  brought  Scandinavian 
bowlders  to  the  eastern  parts  of  England.  On  the  Conti- 
nent the  southern  limit  of  the  ice  was  near  Dresden  and 
Brussels,  and  considerably  north  of  Moscow.  Thus  the  low 
grounds  of  Xorth  Germany  were  covered,  and  the  entire 
Baltic  area.  There  were  several  invasions  and  recessions, 
and  there  were  glaciers  in  the  mountains  of  Wales  and  of 
the  English  lake  district,  and  among  the  Highlands  of  Scot- 
land, long  after  the  ice  melted  off  from  the  lowlands  of 
Great  Britain.  Scandinavia  remained  long  a  region  of 
great  ice  fields,  and  the  glaciers  of  Xorway  are  the  surviv- 
ing remnant  of  her  early  ice  sheets.  The  plains  of  Russia 
were  covered  far  to  the  east,  but  Siberia,  like  Alaska,  seems 
largely  to  have  escaped. 

Reference  has  been  made  to  glacial  extension  in  the 
Alps.  The  range  was  mantled  with  ice  and  snow,  and  sent 
its  glaciers  out  upon  the  plains  of  Bavaria  about  Munich, 
westward  upon  the  flanks  of  the  Jura,  and  as  far  as  Lyons 
in  the  Rhone  Valley  and  southward  upon  the  plains  of 
Italy,  where  now  vast  moraines,  in  some  cases  1,500  feet 
high,  testify  to  the  magnitude  of  the  glaciers  that  occupied 
the  southern  valleys  of  the  Alps.  Similarly,  there  was  great 
extension  of  the  glaciers  of  the  Pyrenees,  Carpathians,  Cau- 
casus, and  Himalayas.  The  same  is  true  of  the  high  Andes 
and  of  the  southern  extremity  of  South  America,  also  of 
the  mountains  of  New  Zealand  and  Australia.  Much  re- 


450  GEOLOGY 

mains  to  be  known  of  these  remote  areas  of  former  glacia- 
tion  in  Asia  and  the  Pacific,  as  well  as  of  the  present  con- 
dition and  extension  of  the  antarctic  ice  fields. 

398.  Causes  of  glacial  climate.— Much  has  been  written 
on  this  subject,  but  definite  additions  to  our  knowledge 
are  as  yet  small.  Both  astronomical  and  geographical 
changes  have  been  thought  to  be  instrumental,  and  it  may 
be  that  the  true  cause  lies  in  the  union  of  both.  Some 
astronomical  theories  may  be  dismissed.  Such  are  the 
following  :  That  glacial  climate  is  due  to  variations  of  the 
sun's  heat  from  time  to  time ;  that  there  are  great  varia- 
tions in  the  temperature  of  the  spaces  traversed  by  the 
solar  system  ;  that  the  position  of  the  poles  has  materially 
shifted ;  and  that  glaciation  is  due  to  the  cooling  of  the 
planet. 

Croll's  theory,  as  it  is  called,  still  requires  attention 
and  is  held  by  many,  or  is  at  least  believed  to  contain  a 
part  of  the  truth.  For  a  full  account  of  it  the  student 
must  consult  the  larger  text-books  or  special  works.  The 
theory  rests,  in  brief,  upon  changes  in  the  eccentricity  or 
the  elongation  of  the  earth's  orbit  around  the  sun,  and 
upon  the  precession  of  the  equinoxes.  We  now  have  sum- 
mer when  the  earth  is  farthest  from  the  sun,  because  the 
northern  hemisphere  is  inclined  to  the  sun  in  that  part  of 
the  earth's  orbit.  But  it  results  from  the  precession  of 
the  equinoxes  that  10,500  years  ago  there  was  summer  in 
the  northern  hemisphere,  when  the  earth  was  nearest  the 
sun,  and  winter  while  the  earth  was  passing  through  the 
remoter  and  longer  part  of  its  orbit— that  is,  the  summer 
was  short  and  hot  and  the  winter  long  and  cold.  Let  this 
fact  be  first  understood.  Now  consider  that  at  somewhat 
irregular  intervals  of  tens  or  even  hundreds  of  thousands 
of  years  the  earth's  orbit  is  greatly  stretched  out,  so  that 
the  earth  is  14,000,000  miles  nearer  the  sun  in  one  part  of 
the  year  than  at  another.  When  winter  occurs  at  the 
greater  distance,  it  will  be  very  long  and  very  cold,  and 


CBNOZOIC  ERA 


451 


the  summer  very  short.  Thus  it  is  thought  that  the  north- 
ern and  southern  hemispheres  would  tend  toward  glacial 
conditions  as  they  alternately  came  to  these  severe  winters. 
The  author  of  the  theory  urges  other  considerations  which 
can  not  be  included  here.  The  evidence  for  glacial  and 
inter-glacial  epochs  indicates  periodicity  in  glaciation,  and 
this  is  favorable  to  an  astronomical  cause.  So  too  are  the 
successive  moraines  of  recession,  indicating  periodical 
pauses  in  the  melting  off  of  an  ice  sheet.  But,  on  the 
other  hand,  we  know  of  but  one  great  glaciation  in  the 
earth's  history,  and  on  the  astronomical  theory  the  ice  age 
should  have  been  several  times  repeated.  The  undoubted 
evidence  of  glaciation  at  the  close  of  Paleozoic  time  in 
India  and  Australia  does  not  essentially  modify  this  objec- 
tion. Nor  have  we  evidence  of  an  alternation  of  invasions 
in  the  north  and  south  hemispheres,  as  the  theory  seems  to 
require. 

For  these  and  other  reasons  many  geologists  hold  to 
geographical  causes.  Such  possible  causes  are  redistri- 
bution of  land  and  water,  changes  in  the  direction  of  the 
ocean  currents,  and  especially  great  increase  in  the  height 
of  the  lands.  We  have  already  seen  how  widely  the  tem- 
perate latitudes  of  eastern  America  and  western  Europe 
differ  through  the  effects  of  the  Gulf  Stream  upon  the 
latter  ;  and  we  know  how  rapidly  the  conditions  of  climate 
change  as  we  go  from  low  to  high  grounds.  In  compara- 
tively late  geological  times  the  ocean  waters  covered  the 
present  Isthmus  of  Panama,  and  apparently  swept  across 
the  Mediterranean  region  and  southern  Asia  in  free  course 
around  the  world.  Late  geological  time  has  seen  Great 
Britain  joined  to  the  continent  by  elevation,  and  diminished 
to  a  group  of  small  islands  by  submergence. 

Now  we  have  repeated  occasion  to  observe  that  North 
America  and  Europe,  especially  toward  the  north,  were 
areas  of  great  elevation  in  the  closing  stages  of  the  Ter- 
tiary and  far  into  the  Glacial  epoch  of  the  Quaternary  pe- 


452  GEOLOGY 

riod.*  The  most  impressive  proof  of  this  is  in  the  fiords  or 
drowned  valleys  of  both  American  coasts,  and  of  Scotland, 
Norway,  and  other  parts  of  northern  Europe.  This  great 
elevation  is  believed  by  many  to  be  the  chief  cause  of  the 
glacial  refrigeration  of  climate.  If  most  of  the  precipitated 
moisture  came  down  as  snow  rather  than  rain,  with  short 
summers,  for  melting,  the  reign  of  the  ice  would  begin. 
Once  started,  glacial  conditions  tend  to  perpetuate  them- 
selves by  chilling  the  atmosphere  and  condensing  the 
vapors  of  the  melting  season  into  a  mantle  of  clouds. 

399.  Duration  and  date  of  the  Glacial  epoch. — The  suc- 
cession of  ice  invasions  proves  impressively  the  great  length 
of  the  epoch.  Historical  measures  of  time  are  small  as 
compared  with  those  needed  for  the  ice  to  advance  from 
the  heart  of  British  America  to  the  Missouri  River,  and 
melt  off  again.  And  even  advocates  of  the  essential  unity 
of  the  glacial  invasion  admit  the  immense  time  required 
for  fluctuations  along  the  margin  of  the  ice,  and  for  the 
successive  deposits  of  the  many  moraines  of  recession. 

Xo  satisfactory  computations  as  to  the  lapse  of  time 
since  the  ice  departed  have  been  made.  The  last  period  of 
great  eccentricity  of  the  earth's  orbit  was  about  70,000 
years  ago.  Those  who  accept  Croll's  theory  would  there- 
fore be  disposed  to  ascribe  such  an  antiquity  to  the  inva- 
sion. But  many  hold  the  post-Glacial  epoch  to  be  much 
shorter.  As  has  been  stated,  it  is  known  that  the  present 
Niagara  began  its  work  after  the  ice  had  left  the  Mohawk 
Valley.  Computations  based  on  the  present  rate  of  gorge- 
cutting,  or  recession  of  the  falls,  have  led  to  view  that  the 
7  miles  of  the  gorge  have  been  made  in  7,000  to  8,000  years. 
In  harmony  with  this,  similar  results  are  given  from  the 
recession  of  the  Falls  of  St.  Anthony,  at  Minneapolis.  And 


*  Reference  should  here  be  made  to  the  elaborate  argument  of 
Suess  (French  edition  I^a  Face  de  la  Terre)  that  such  changes  are  due 
to  oscillations  of  the  sea  rather  than  of  the  land. 


CENOZOIC 


453 


it  is  claimed  that  many  topographic  forms  built  of  loose 
sands  and  gravels  could  not  have  kept  their  contours  for 
more  than  a  few  thousand  years  of  exposure.  But,  on  the 
other  hand,  there  are  strong  reasons  for  thinking  that  the 
erosion  of  the  Niagara  gorge  has  not  been  uniform,  and 
that  the  time  is  vastly  greater.  It  should  also  be  remem- 
bered that  the  perishable  hills  to  which  reference  has  been 
made  were  commonly  protected  by  forests  until  recent  cen- 
turies, at  least  in  America.  The  date  of  the  Glacial  epoch 
must  therefore  be  regarded  as  uncertain,  but  there  is  much 
to  encourage  the  belief  that  our  knowledge  will  become 
more  definite. 

400.  General  advantages  of  glaciation. — In  all  the  great 
lakes  which  were  caused  by  the  melting  of  the  ice  and  the 
retention  of  the  waters  fine  offshore  muds  were  spread  over 
wide  areas.  These  form  valuable  soils,  and  are  found  in 
Minnesota,  Dakota,  and  Manitoba,  along  the  bottoms  of 
Lake  Agassiz,  and  also  bordering  the  Great  Lakes  on  all  the 
lower  grounds,  as  about  Lake  Erie  in  Ontario  and  Ohio,  and 
south  of  Lake  Erie  and  Lake  Ontario  in  Xew  York.  The 
valley  of  Utah  also  is  floored  in  the  same  manner,  and 
shows  the  greatest  fertility  under  irrigation. 

Professor  Shaler  has  called  attention  to  the  renewal  of 
all  soils  that  were  subjected  to  the  glacial  plow  and  received 
accessions  of  coarser  mechanically  derived  materials.  These, 
he  affirms,  will  long  continue  to  yield  to  plants  the  ele- 
ments of  nutrition  by  constant  disintegration,  while  resid- 
ual soils  will  become  poor,  after  their  quota  of  vegetable 
matter  is  exhausted.  The  sum  total  of  water  power  of  the 
Northern  States  has  been  vastly  increased,  because  most 
streams  have  by  glaciation  been  set  to  flow  at  higher  levels. 
Rapids  and  waterfalls  are  common  on  many  streams  which 
before  the  ice  invasion  moved  sluggishly  along  near  base 
level.  The  increased  variety  of  natural  scenery  has  already 
received  mention,  and  reference  has  been  made  to  the 
gorges  and  waterfalls  as  chiefly  of  glacial  origin.  The  same 
30 


454  GEOLOGY 

principle  receives  perhaps  its  fullest  illustration  in  the 
thousands  of  beautiful  lakes  which  serve  as  places  of  resort, 
afford  healthful  recreation  in  many  forms,  and  more  and 
more  furnish  to  the  cities  and  towns  supplies  of  pure 
water. 

LIFE  IN  THE  QUATERNARY  EPOCH 

401.  Migrations  caused  by  ice  invasions. — These  took 
place  extensively  not  only  because  the  ice  actually  occupied 
the  ancient  habitat  of  many  groups,  but  because  it  chilled 
the  climate  of  adjacent  regions,     forests  of  particular  spe- 
cies were  crushed  or  were  dying  off  on  one  side  and  slowly 
advancing  on  the  other.     Or  they  might  be  destroyed  alto- 
gether if  the  advance  was  too  rapid  and  the  climatic  strain 
on  the  vitality  of  the  species  was  too  great.     In  many  cases 
such  forced   migrations   would    modify   the   species,  and 
change  the  proportions  of  the  various  kinds  which  made 
up  a  fauna  or  a  flora.     When  the  ice  retreated  the  groups 
would  in  some  measure  push  back  and  recover  their  terri- 
tory.    Some  arctic  forms  did  not  go  back,  but  found  con- 
genial homes  on  the  mountain  tops.     Plants  and  insects 
thus  gained  a  place  among  the  Alps,  or  even  the  summits 
of  the  White  Mountains  or  the  Adirondacks,  where  they 
are  found  to-day,  far  separated   from  their  kind.     Other 
heights,  outside  of  the  glacial  territory,  do  not  possess  these 
arctic  forms. 

MAMMALIAN  LIFE  IN  THE  QUATERNARY  PERIOD 

402.  The  Tertiary  marine  invertebrates  are  more  and 
more  like  those  of  the  present,  and  the  same  is  therefore 
true  of  Quaternary  times.     These  forms  therefore  need  not 
concern  us  here.     But  the  mammalian  species  of  the  early 
Quaternary  are  distinct  from  those  of  the  Tertiary  and  are 
in  their  turn  extinct.     Some  were  of  great  size,  and  wan- 
dered over  continents  where  none  of  their  modern  relations 
are  found.     With  them  man  himself  appeared  on  the  scene, 


CENOZOIC  EEA 


455 


at  the  summit  of  the  series  of  living  forms,  and  about  to 
become  the  master  of  the  organic  and  the  inorganic  world. 
The  most  conspicuous  Quater- 
nary creatures  of  Xorth  America 
were  the  elephants.  They  were 
much  larger  and  heavier  than 


FIG.  288. -Tooth  of  Quaternary 
elephant  (|  natural  size). 


Fto.  289.— Tooth  of  mastodon. 


modern  elephants,  and  ranged  over  the  entire  United 
States  and  parts  of  Canada  and  Alaska.  The  mastodons 
also  came  over  from  the  Tertiary  (with  new  species,  how- 


FK;.  290.—  Megatheriun 


ever),  and  have  also  a  wide  distribution.     They  have  been 
sometimes  preserved  by  miring  in  soft  grounds,  and  there 


456  GEOLOGY 

have  been  found  in  connection  with  their  skeletons  masses 
of  grass  and  herbs  which  the  creature  had  devoured,  but 
had  not  digested.  The  skeleton  of  an  American  mastodon 
now  in  the  British  Museum  measures  17  feet  in  length  and 
11  feet  in  height.  There  were  also  stags  and  buffaloes  of 
great  size,  and  many  horses.  A  few  remains  of  the  saber- 


Fio.  291.— Extinct  armadillo,  Glyptodon. 

toothed  tiger  and  the  bear  have  been  found  in  the  South 
and  West,  but  the  most  of  North  American  mammals  lived 
on  vegetation. 

In  South  America  a  large  Quaternary  fauna  has  passed 
away.  The  dominant  forms  are  known  as  edentates,  some  of 
them  gigantic  creatures,  but  in  form  like  sloths  and  armadil- 
los. One  species  is  Megatherium  (great  beast),  sometimes 
nearly  20  feet  long,  with  massive  body  and  thick,  clumsy 
legs  and  tail.  Other  edentates  (Glyptodon)  had  an  immense 
shelly  armor,  much  resembling  the  carapace  of  a  turtle. 

The  European  mammals  are  of  great  number  and  often 
also  of  large  size.  They  include  many  forms  now  confined 
to  warmer  climates,  and,  on  the  other  hand,  animals  like 
the  reindeer  occur  far  south,  bearing  unmistakable  witness 
to  great  alternations  of  climate.  European  mammalian 
bones  have  been  abundantly  found  in  the  deposits  of  rivers 
and  in  caverns.  The  latter  were  often  the  haunts  of  wild 
animals,  as  the  hyenas,  and  their  bones  with  those  of  their 
prey  are  found  together.  The  Irish  elk  is  sometimes 


CENOZOIC  ERA 


45T 


found  in  great  and  perfect  skeletons  in  peat  bogs  in  which 
the  creature  had  sunk.  One  specimen  has  a  spread  of 
antlers  of  12  feet.  A  single  cavern  in  England  afforded 
bones  of  hyena,  elephant,  rhinoceros,  hippopotamus,  cave 


FIG.  292.-Skeleton  of  the  Irish  elk. 


lion,  brown  bear,  and  many  other  species.  The  elephant 
or  mammoth,  as  we  have  seen,  was  almost  everywhere  in 
J^orth  America,  and  ranged  through  northern  Europe 
and  northern  Asia,  indicating  a  common  highway  of  mi- 


458  GEOLOGY 

gration  across  the  Bering  Straits  region  in  the  time  of 
continental  elevation.  Perfect  specimens  with  flesh  and 
covering  of  wool  and  long  hair  have  been  found  frozen  into 
the  ice  of  the  Lena  River.  The  dogs  ate  the  flesh  of  this 
creature  of  an  extinct  species,  and  the  tusks  have  been 
sought  for  export,  and  thus  have  made  their  contribution 
to  the  trade  in  ivory.  The  length  of  one  of  these  Siberian 
mammoths  was  16£  feet  and  the  height  9£  feet.  The  dis- 
tribution of  mammals  in  Europe  proves  land  connection 
and  freedom  of  migration  between  England  and  the  Con- 
tinent, and  between  southern  Europe  and  Africa.  The 
reindeer  at  one  time  grazed  southward  as  far  as  the  Alps 
and  the  Pyrenees.  The  existence,  however,  of  lions,  ele- 
phants, hippopotami,  and  other  forms  far  north  in  Europe 
and  Asia,  indicates  a  warmer  climate  than  the  present  as 
having  prevailed.  Even  the  warm-clad  mammoth  could  not 
now  live  in  the  far  north  of  Siberia. 

403.  Man. — Much  remains  to  be  learned  of  the  advent 
of  man  upon  the  planet.  The  broad  facts  of  our  present 
knowledge  are  that  the  human  race  has  great  antiquity  in 
comparison  with  the  historical  period,  and  that  the  primi- 
tive man  lived  as  a  rude  savage,  and  was  a  contemporary  of 
several  species  of  mammals  which  are  now  extinct.  Our 
knowledge  of  man's  antiquity  depends  upon  several  branches 
of  science,  particularly  upon  history,  philology,  anthro- 
pology, archaeology,  and  geology.  History  carries  us  to 
the  earliest  extant  records,  as  of  Egypt.  Philology  and 
anthropology  require  high  antiquity  for  the  development 
of  languages  and  races,  for,  whatever  views  are  taken  of 
man's  origin,  it  is  agreed  that  he  began  as  one  type  in  a 
single  locality.  Archaeology  finds  its  records  in  objects 
made  or  used  by  man,  and  draws  a  picture  of  his  life. 
Geology,  with  paleontology,  refers  the  ancient  man  to  his 
place  in  the  chain  of  life,  amid  the  succession  of  physical 
events.  The  antiquity  of  the  primitive  man  must,  there- 
fore, be  determined  mainly  by  our  science. 


CENOZOIC  ERA 


459 


It  has  been  agreed  to  call  the  earliest  man  Paleolithic 
(ancient  stone),  because  he  used  the  rudest  and  roughest 
forms  of  stone  implements.  They  were  chipped,  but  never 


Kiu.  '493. — Paleolithic  drawing  of  a  mammoth  on  a  surface  of  horn. 

polished.  Here  the  reference  is  chiefly  to  Europe,  because 
in  America  the  age  of  chipped  stone  continued  until  the 
retirement  of  the  Indians  before  the  early  settlers.  Paleo- 


Fio.  294.— (Supposed)  Paleolith  found  in  New  Jersey.    Shows  the  general  character 
of  Paleolithic  flints. 

lithic  man  lived  often  in  caverns  and  subsisted  upon  berries 
and  such  fishes  and  land  animals  as  he  could  contrive  to 
catch.  His  implements,  and  rarely  his  bones,  are  found  in 


460  GEOLOGY 

some  caverns  of  England,  France,  Belgium,  and  elsewhere 
in  Europe,  in  association  with  the  bones  of  elephants,  lions, 
bears,  hyenas,  and  other  extinct  animals.  Many  flint  imple- 
ments have  been  found  in  the  terraces  of  rivers,  as  in  north- 
ern France.  The  situation  is  such  as  to  prove  deposit 
closely  following  an  ice  invasion.  The  more  ancient  of  the 
relics  of  man  in  Europe  are  believed  to  date  from  an  inter- 
glacial  epoch,  for  these  are  earlier  than  the  reindeer  man  of 
southern  France  who  belongs  to  the  later  great  glaciation 
of  central  Europe.  These  later  Paleolithic  remains  (some- 
times called  Mesolithic)  are  more  advanced,  and  include 
quite  skilled  drawings  of  the  mammoth  on  pieces  of  ivory. 
No  undisputed  Paleolithic  remains,  associated  with  glacial 
deposits,  have  been  found  in  America.  Chipped  implements 
found  at  Trenton,  N.  J.,  and  in  the  drift  of  Ohio  and 
Indiana,  have  been  held  to  be  true  Paleoliths,  or  represent- 
atives of  the  earliest  man,  but  the  evidence  is  deemed 
insufficient  by  many. 

It  is  agreed  by  the  highest  authorities  that  the  skulls  of 
Paleolithic  men  thus  far  found  are  truly  human,  and  are 
not  intermediate  between  man  and  any  lower  form.  Stu- 
dents of  geology  should  know,  however,  that  if  a  "  missing 
link  "  were  found,  it  would  not  be  a  creature  between  man 
and  an  ape,  but  between  man  and  some  ancestral  type  of 
long  ago.  In  this,  as  in  all  questions,  the  truth  should  be 
sought  without  prejudice  or  fear.  The  antiquity  of  man 
and  his  possible  evolution  from  lower  forms  of  life  are 
questions  of  science.  Xo  answer  which  science  may  render 
is  inconsistent  with  the  highest  views  of  our  origin  and 
destiny. 

Some  relics  have  been  supposed  to  indicate  a  Tertiary 
man,  but  the  general  verdict  on  this  subject  is  "  not  proved." 
It  could  hardly  be  useful  to  state  figures  in  relation  to  the 
age  of  the  race.  They  would  be  only  conjectural,  and  it  is 
better  to  wait  for  more  knowledge.  If  the  time  of  the 
Glacial  epoch,  and  of  its  various  subdivisions,  becomes  bet- 


CENOZOIC  ERA  4.51 

ter  known,  real  light  may  be  thrown  upon  the  other  ques- 
tion, which,  as  we  now  see,  is  closely  related  to  it. 

Following  Paleolithic  is  the  Neolithic  man.  He  used 
implements  of  polished  stone  and  of  bone,  often  very  skill- 
fully and  perfectly  made.  Many  of  the  animals  found  with 
Paleolithic  remains  had  now  become  extinct,  and  some  ani- 
mals, such  as  goats,  oxen,  sheep,  and  dogs,  were  domesti- 
cated. Cereals  were  also  cultivated.  The  Kitchen  Mid- 
dens or  primitive  shell  heaps  of  the  Baltic  shores,  and  the 
Lake  Dwellings  of  Switzerland,  belong  to  the  Neolithic 
times.  Then  followed  the  Bronze  and  Iron  ages,  as  they 
were  called.  These  were  not  contemporaneous  in  different 
regions,  and,  like  the  Neolithic,  belong  more  to  archaeology 
and  less  to  geology  than  is  the  case  with  the  evidence  for 
Paleolithic  man. 

It  should  be  remarked  that  since  man  attained  any  num- 
bers, he  has  been  a  powerful  agent  for  the  redistribution 
and  even  for  the  extinction  of  many  groups  of  animals  and 
plants,  as  well  as  for  actual  modifications  of  the  earth's 
surface. 

404.  Geological  time.— Our  study  would  have  been  to 
little  purpose  if  it  were  now  needful  to  urge  that  the 
history  of  the  earth  is  long.  But  a  brief  notice  of  the 
opinions  of  geologists  on  this  subject  will  be  suitable  at 
this  point.  We  may  observe  that  physicists  and  astrono- 
mers are  inclined  to  shorten  the  estimates  of  geological 
time  on  the  basis  of  the  rate  of  cooling  and  other  consid- 
erations, and  to  place  a  limit  of  ten  or  twenty  million  years 
for  the  interval  from  the  first  nebula  to  the  present.  Geol- 
ogists would  all  consider  this  too  short  in  the  light  of  phys- 
ical changes  and  of  a  great  number  of  organic  revolutions. 
The  making  and  unmaking  of  hundreds  of  rock  formations, 
and  the  inscription  upon  the  face  of  the  continents  of  many 
successive  topographies,  are  the  constant  material  of  geo- 
logical study.  The  student  will  perhaps  gain  the  most  real 
and  serviceable  appreciation  of  the  enormous  duration  of 


462  GEOLOGY 

terrestrial  history  if  he  will  think  of  many  single  phases, 
each  one  of  which  demands  long  duration.  He  may  take, 
for  example,  any  one  of  the  Paleozoic  limestones,  the  mak- 
ing of  a  single  thick  bed  of  coal,  the  accumulation  of  the 
English  chalk,  the  subduing  of  the  Appalachian  moun- 
tains to  the  Cretaceous  peneplain,  the  several  glacial  and 
inter-glacial  phases  of  the  Glacial  epoch,  the  formation 
of  great  coral  reefs,  or  the  making  of  large  modern  del- 
tas. And  behind  all  this  is  the  expanse  of  pre-Paleozoic 
time,  believed  by  Dana  to  be  longer  than  all  time  from  the 
opening  Paleozoic  until  to-day.  Such  considerations  influ- 
ence one's  thought  much  more  than  figures  can  do.  But 
even  figures  based  on  rational  estimates  are  not  without 
value.  On  the  basis  of  the  average  rate  of  denudation 
and  sedimentation,  Upham  arrives  at  28,000,000  years  for 
all  time  from  the  beginning  of  the  Paleozoic  era.  There 
are  many  possibilities  of  error  in  such  an  estimate.  The 
rapidity  of  denudation  depends  on  climate,  the  height  of 
the  lands,  the  acids  of  the  atmosphere,  and  perhaps  other 
factors.  The  lack  of  a  covering  of  plants  may  have  made 
the  destruction  of  the  lands  in  early  periods  much  more 
swift  than  in  later  time.  Dana  estimates  Paleozoic,  Meso- 
zoic,  and  Cenozoic  time  in  the  ratio  of  12  :  3  :  1.  Reckon- 
ing 36,000,000  years  for  Paleozoic  time,  he  finds  36,000,000 
+  9,000,000  +  3,000,000  =  48,000,000  years,  which  he  holds 
to  be  less  than  half  the  period  of  the  earth's  whole  develop- 
ment. Dr.  James  Croll  estimates  72,000,000  years  for  the 
sedimentary  rocks;  Wallace,  on  the  other  hand,  assumes 
28,000,000  years  for  the  same.  This  diversity  does  not 
mean  that  our  knowledge  is  of  no  value,  for  all  these  mag- 
nitudes are  of  the  same  order,  and  the  lowest  are  as  far  be- 
yond our  imagination  as  the  highest.  Vast  duration  is  the 
verdict  of  all  who  know  the  facts  that  must  be  reckoned 
with.  Xot  many  students  of  the  earth  could  be  found  who 
would  not  accept  Walcott's  statement  that  the  history  is  to 
be  reckoned  by  tens,  not  by  hundreds,  of  millions  of  years. 


CENOZOIC   ERA 


463 


To  gain  in  any  measure  this  conception  of  the  immensity 
of  time  is  one  of  the  highest  rewards  of  study. 

405.  Orderly  progress  of  the  earth's  history.— Progressive 
unfolding  has  been  the  law  throughout.  We  began  our 
study  of  Part  III  with  the  affirmation  of  two  great  lines  of 
evolution,  the  geographical  and  the  organic.  The  conti- 
nents began  with  straggling  and  isolated  lands,  and  grew 
and  consolidated  by  successive  deposit  and  uplift.  De- 
pressions have  intervened,  but  the  goal  has  not  been  ob- 
scured. Progress  has  usually  been  quiet,  but  not  infre- 
quently energy  has  gathered,  until  vast  and  almost  catas- 
trophic changes  followed  in  quick  succession.  Amid  every 
diversity  of  slow  and  swift,  uplift  and  down  wear,  all  forces 
have  wrought  together  to  make  lands  of  moderate  average 
altitude,  great  areas  with  genial  climate,  rocks  covered  with 
soil,  and  soil  supporting  abundant  life. 

Equally  wonderful  in  its  majestic  ongoing  has  been  the 
progress  of  life.  From  the  earliest  fossil-bearing  rocks  to 
the  last  sands  laid  on  the  beach  the  tendency  of  life  has 
on  the  whole  been  upward.  Lowly  forms  have  given  way 
to  higher,  and  clumsy  generalized  types  like  the  early  fishes, 
reptiles,  and  birds,  have  yielded  the  stage  to  nobler  and 
more  special  groups.  The  land  forms  came  last,  but  stead- 
ily gained  in  numbers,  variety,  and  physical  rank,  until 
signs  of  intelligence  appeared,  and  these  received  their 
crown  in  man. 


INDEX 


Aar  Glacier,  92,  95,  99-106. 
Acadian  epoch,  310. 
Acidic  rocks,  207. 
Actinolitc,  195. 
Adirondacks,  256,  262,  305. 

dike  in,  245. 

lakes  of,  276. 

rivers  of,  284. 
Adjustment  of  rivers,  283. 
Agassiz,  A.,  182. 
Agassiz,  L.,  91,  92,  96,  105,  432. 
Agassiz,  Glacial  Lake,  440. 
Agriculture,  geological  effects  of,  186, 

187. 

Alabaster,  197. 
Alaska,  glaciers  of,  93, 106-109. 

placers  in,  240. 
Algonkian  era,  302. 
Alluvial  cones,  57, 187. 
Alps  (see  also  Switzerland),  256,  258, 
261. 

avalanches  in,  109. 

building  of,  417. 

glaciers  of,  90. 

former  glaciers  of,  448. 

St.  Gothard  Tunnel  in,  186. 
Aluminum,  192. 
Amber,  insects  in,  420. 
Amethyst,  194. 
Amherst  Museum,  380. 
Ammonites,  389,  408. 
Amorphous,  199. 
Amphibians,  374,  391. 
Andes,  256. 


Animals,  geological  effects  of  marine, 
175. 

geological  work  of  land,  183. 

migration  of,  183. 
Antecedent  streams,  285. 
Anticlinal  fold,  225. 
Antiquity  of  man,  424,  457. 
Anthracite,  196,  214,  410. 
Anthropic  geology,  185. 
Apatite,  307. 
Appalachian,  255,  261,  305. 

Revolution,  375. 
Archaean  era,  302. 
Archseopteryx,  396. 
Arizona,  sand  storms  in,  6. 

silicifled  wood  in,  174. 
Arkansas,  weathered  limestone  in,  21. 

Hot  Springs  in,  81. 
Artesian  wells,  83. 
Asaphus,  329. 
Asbestos,  195. 
Astarte,  387. 
Asteroids,  347. 
Atlantosaurus,  393. 
Atmosphere,  18. 
Atolls,  180. 
Atrypa,  337,  348. 
Aubrey  sandstone,  366. 
Ausable  Chasm,  43. 
Australia,  corals  in,  178, 180. 
Avalanches,  109. 
Avicula,  339. 
Aviculopecten,  350. 
Azoic,  303. 

465 


466 


.  GEOLOGY 


Baculites,  408. 
Bad  Lands,  34,  417. 
Barnacle,  329. 
Barrier  reefs,  180. 
Basalt,  139,  207,  209,  248. 
Base  level,  281. 
Basic  rocks,  207. 
Beaches,  124. 

elevated,  164. 
Beavers,  work  of,  184. 
Bed,  215. 

Beekmantown  limestone,  319. 
Belemnites,  389,  408. 
Bellerophon,  326. 
Bermudas,  eolian  formations  in,  11, 14, 

179. 

Birdseye  limestone,  320. 
Birds,  Mesozoic,  396,  410. 
Bituminous  coal,  196. 
Black  Hills,  312,  382. 
Black  River  limestone,  320. 
Black  shale,  344. 
Blastoids,  326,  347,  373. 
Blue  Ridge,  256,  262. 
Bog  ore,  198. 
Bomb,  volcanic,  141. 
Bonanza,  239. 
Bonneville,  Lake,  279,  443. 
Bowlder  clay,  428. 
Bowlders,  erratic,  429. 
Brazil,  weathering  in,  32. 
Breccia,  203. 
Brachiopods,  298. 

Cambrian,  313. 

Devonian,  348. 

Lower  Silurian,  326. 

Meaozoic,  387. 

Upper  Silurian,  337. 
Brontosaurus,  393. 
Bronze  age.  460. 
Buhrstone  formation,  415. 
Building  stones,  Cambrian,  317. 

Devonian,  356. 

Lower  Silurian,  331. 

Mesozoic,  397. 

Upper  Silurian,  340. 

weathering  of,  30. 


Cadell,  H.  M.,  225. 

Cairngorm,  194. 

Calamites,  369,  372. 

Calciferous  epoch,  319. 

Calcite,  196. 

Calcium,  192. 

California,  Great  Valley  of,  383. 

irrigation  in,  187. 

Mount  Shasta  in,  264. 

sand  storms  in,  6. 

Sierran  Range  in,  383. 

Table  Mountain  in,  280. 
Calymene,  328,  338. 
Cambrian  period,  310. 
Camptosaurus,  393. 
Cannel  coal,  197. 
Cape  Cod,  4,  5,  8,  10. 
Carbon,  192. 
Carbonates,  196. 
Carboniferous  period,  359. 

North  America  in,  360. 

vegetation  of,  368. 
Carnivores,  421. 
Caspian  Sea,  114. 
Catskill  Mountains,  47,  254,  344. 
Caverns,  84,  123,  455. 
Cellular  structure,  200. 
Cenozoic  era,  411. 
Cephalaspis,  355. 
Ccphalopods,  Devonian,  849. 

Mesozoic,  388. 

Lower  Silurian,  327. 
Ceratites,  387. 
Chain  Corals,  324,  336,  346. 
Chalk,  205,  406. 
Chamberlin.  T.   C.,  83,  106,  302,  436, 

437,  447. 
Champlain,  Lake,  114. 

depression,  443. 

Charleston  earthquake,  154,  160. 
Chattahoochee  formation,  415. 
Chazy  epoch,  320. 
Cliemung  epoch,  844. 
Chert,  194. 
Chesapeake  Bay.  166. 

formation,  415. 
Chester  group,  361. 


INDEX 


467 


Chicago,  artesian  waters  in,  83. 

Drainage  Canal,  187,  441. 

dunes  near,  12. 

Niagara  limestone  near,  335. 
Chifola  formation,  415. 
Chili,  earthquake  in,  160. 
China,  loess  of,  13. 
Chlorite,  195. 
Chute,  239. 

Cincinnati  anticline,  356. 
Claiborne  formation,  415. 
Clarke,  J.  M.,  321,  342,  345. 
Clay,  204. 

deep-sea,  132. 
Clay-ironstone,  222. 
Cleavage,  193,  241. 
Clitfs,  shore,  123. 
Climate,  glacial,  449. 

modified  by  lakes,  112. 
Clinton  epoch,  335. 
Coal,  carboniferous,  359. 

kinds  of,  196. 

Laramie,  409. 

mode  of  occurrence,  362. 

Richmond,  397. 

Tertiary,  424. 

vegetable  origin  of,  366. 

measures,  360,  361. 
Coast  Range,  255,  415. 
Colgate  University,  Hamilton  forma- 
tions at,  343. 

Color  of  rocks,  28, 175,  201,  205. 
Colorado,  altitude  of,  53. 

ancient  glaciers  of,  434. 

avalanches  in,  109. 

beaver  work  in,  184. 

coal  of,  214. 

fossil  reptiles  in,  393. 

Front  Range  in,  382. 

hot  springs  in,  81. 

irrigation  in,  187. 

mountains  of,  260. 

ores  of,  239. 

parks  of,  260. 

Seven  Lakes  in,  273. 

unconformity  in,  229. 

weathering  in,  19, 23, 26,  29, 31, 33, 34. 


Colorado  River,  271. 

canon,  270. 

Colorado  formations,  402. 
Color  of  rocks,  28,  175,  201,  205. 
Columnar  structure,  248. 
Comanche  Series,  402. 
Compact  rocks,  199. 
Concretion,  222. 
Cones,  alluvial,  57,  187. 

volcanic,  142,  243,  263. 
Conglomerate,  203. 
Conifers,  372,  385. 
Connecticut,  lava  sheets  of,  244. 

mud  cracks  in,  221. 

Triassic  in,  380. 

River,  41,  46,  60,  66. 
Consequent  streams,  284. 
Continental  shelf,  287. 
Continents,  286. 
Conularia,  328. 
Cope,  E.  D.,  394. 
Copper,  239,  317. 
Corals,  carboniferous,  373. 

Devonian,  346. 

distribution  of,  178. 

Lower  Silurian,  324. 

Mesozoic,  385. 

rate  of  growth  of,  182. 

reefs,  179. 

Upper  Silurian,  337. 
Corniferous  epoch,  343. 
Coulter,  J.  M.,  187. 
Crater  lakes,  276. 
Creep  in  weathering,  29. 
Creodonts,  421. 
Cretaceous  period,  398. 
Crinoids,  Carboniferous,  373 

Devonian,  351. 

Silurian,  325.  337. 
Crioceras,  408. 
Croll's  theory,  449. 
Cross  bedding,  219. 
Crust  movements,  154. 
Crustacea,  328. 
Cup  corals,  324. 
Currents  of  ocean,  118. 
Cuttlefish,  389. 


468 


GEOLOGY 


Cycads,  385. 

Cycle  of  erosion,  281. 

Cystids,  313,  325,  347. 

Dana,  J.  D.,  49,  52,  76,  79,  150,  177, 

211,  231,  264,  287,  345,  461. 
Dakota  formation,  402. 
Danube,  51. 
Darton,  N.  H.,'  234. 
Darwin,  15, 167, 177,  183,  285. 
Davis,  W.  M.,  55,  58,  79,  272,  380,  415. 
Dead  Sea,  114,  274. 
Deformation,  168. 
Delaware  Kiver,  70,  285. 
Delaware  Bay,  166. 
Deltas,  66. 

in  lakes,  112. 

Mississippi,  167. 
Denmark,  dunes  of,  12. 
Denudation,  32,  52. 
Deposition,  4, 131. 
Devonian  period,  342. 

North  America  in,  355. 
Diabase,  208. 
Dicellomus,  313. 
Dikes,  18,  73,  246. 
Diluvial  theory,  431. 
Dinichthys,  354. 
Dinosaurs,  392. 
Dinotherium,  421. 
Diorite,  208. 
Dip,  227. 

Diplograptua,  322. 
Discina,  326,  348. 
Distribution  of  species,  299. 
Divides,  286. 
Dolomite,  196. 
Dolomites,  the,  259. 
Drainage,  Canal,  441. 

evolution  of,  283. 

post-glacial,  444. 

trellised,  285. 
Drift,  the,  426. 
Driftless  area,  446. 
Drowning  of  valleys,  283. 
Drumlins,  268,  269. 
Dunes,  8. 


Dunes,  in  arid  regions,  13. 

in  Bermudas,  11. 

in  shore  regions,  10. 

migration  of,  10. 
Dynamical  geology,  188. 

Earth,  form  of,  288. 

interior  of,  289. 
Earthquakes,  154. 

causes  of,  163. 

Charleston,  154, 160. 

distribution  of,  158. 

geological  effects  of,  159. 

principles,  156. 
Earthworms,  183. 
Edentates,  455. 
Elements,  chemical,  191. 
Elephants,  421,  454. 
Elevation,    movements   of,   164,   165, 

277. 

Elk,  Irish,  174,  456. 
Emerson,  B.  K.,  380. 
England,  dunes  of,  12. 

fens  of  eastern,  188. 

landslips  in,  79. 

rivers  of,  37. 

rocks  of,  295. 

sea  cliffs  of,  120. 

soils  of,  7. 

springs  in,  81. 
Eocene  epoch,  414. 
Eolian  geology,  3. 
Epicentrum,  156. 
Equivalent  strata,  300. 
Equus,  423. 
Erie  Canal,  186. 
Erosion,  4. 

by  glaciers,  96. 

by  rivers,  41. 

by  the  sea,  119. 

by  underground  waters,  77. 

by  winds,  4. 

cycles  of,  281. 
Eruptive  rocks,  208. 
Eskers,  266. 
Etna,  141, 148. 
Eurypterus,  329,  839,  352. 


INDEX 


469 


Eutaw  beds,  402. 
Evolution,  296,  462. 

Faceted  pebbles,  4. 

Fairchild,  II.  L.,  Ill,  267. 

Fall  line,  73. 

Fan  structure,  262. 

Faults,  232,  381. 

Faunas,  297. 

Favosites,  324,  346. 

Feldspars,  194. 

Ferns,  329,  353,  391,  408. 

Fertilizers,  185, 197,  307,  341,  424. 

Fingal's  Cave,  123,  248. 

Finger  Lakes,  274,  275. 

Fiords,  166. 

Fire  clay,  204. 

Firn,  92. 

Fishes,  329,  853,  391,  408. 

Flint,  194. 

Floods,  58,  62,  112. 

glacial,  438. 
Flood  plains,  58,  62. 
Floras,  297. 
Florida,  altitude  of,  53. 

deposits  of,  414. 

spring  in,  80. 
Focus,  156. 
Folds,  223. 
Foliated,  200. 
Foraminifera,  385. 
Forests,  Carboniferous,  365. 

Cretaceous,  405. 

Devonian,  357. 

plantitis  and  destruction  of,  18^ 

preservation  of,  75. 
Fossils,  220,  292-205. 
Frasrmental  rocks,  201,  215. 
Frinsrinsr  reefs,  180. 
Front  Range,  382. 
Fulgurites,  27. 
Fumarole.  143. 

Fundy,  tides  of  Bay  of,  116, 117. 
Fusulina,  372. 

Gabbro,  208. 
Ganges,  54,  69,  79. 
31 


Gangue,  239. 

Ganoids,  353,  391. 

Garden  of  the  Gods,  31,  382. 

Gases,  volcanic,  137. 

Gastropods,  Lower  Silurian,  327. 

Mcsozoic,  388. 

Tertiary,  419. 

Geikie,  A.,  32,  44, 121, 137, 149,  212. 
Genesee  Kiver,  56,  60,  62,  78,  272. 
Geode,  223. 

Geography,  after  Appalachian  revolu- 
tion, 377. 

Cambrian,  315. 

Carboniferous,  360. 

Cretaceous,  398. 

Devonian,  355. 

Lower  Silurian,  319,  332. 

physical,  190. 

pre-Paleozoic,  306. 

Tertiary,  411. 
Geological  time,  divisions  of,  300. 

extent  of,  460. 
Geology,  departments  of,  1. 

relations  of,  1. 

study  of,  2. 
Georgian  epoch,  310. 
Geysers,  143. 
Giant's  Causeway,  249. 
Gilbert,  G.  K.,  67,  124,  127,  167,  243, 

279,  442. 
Glacial  epoch,  426. 

topography,  266. 
Glaciers,  90. 

melting  of,  101. 

mode  of  formation,  90. 

moraines  of,  98. 

motion  of,  92. 

structure  of,  95. 

transportation  by,  97. 

Viesch,  frontispiece. 
Glauconite,  414. 
Globe,  288. 

Glohiererina  ooze,  131. 
Glyprodon,  455. 
Gneiss,  213.  223. 
Gold,  198,  238,  383,  397. 
Goniatite,  351,  374. 


470 


GEOLOGY 


Graded  slopes,  259. 

Grammy  si  a,  350. 

Granite,  207. 

Granular,  200. 

Graphite,  196. 

Graptolites,  323,  337. 

Gravel,  203. 

Gravitation,  weathering  by,  25. 

Great  Basin;  233,  274,  382,  413. 

Great  Britain,  elevated  beaches  of,  165. 

fossil  reptiles  of,  392. 

Glacial  epoch  in,  448. 

subsidence  in,  167. 
Great  Lakes,  110, 168,  441,  442. 
Great  Plains,  412. 
Great  Salt  Lake,  114,  279. 
Greenland,  98, 106,  167. 
Green  Mountains,  256,  332. 
Green  Kivcr,  200,  285. 
Greensand,  414,  424. 
Gryphaea,  408. 
Guano,  185. 
Gulf  Stream,  118,180. 
Gypsum,  197,  341. 

Hade,  234. 

Hall,  J.,  345. 

Halysite  corals,  324,  336,  346. 

Hamilton  epoch,  343. 

Hamites,  408. 

Hardness,  193. 

Hawaiian  Islands,  149,  179,  193,  264, 

288. 

Helderberg  epoch,  342. 
Hematite,  197. 
Henry  Mountains,  246,  265. 
HilU,  264. 
Himalayas,  417. 
Historical  jreoloary.  1,  291. 
Hitchcock.  E..  380,  431. 
Holland,  dunes  of,  12. 
Holoptychius,  353. 
Homalonotus,  338,  352. 
Hornblende,  195. 
Horse,  422. 
Horse  (in  veins),  288. 
Hot  Springs,  81. 


Hudson  epoch,  821. 
Hudson  Valley,  34,  287. 

fiord  of,  166. 

Palisades  of,  56,  244,  381. 

River,  51,  71. 
Hunius  acids,  20. 
Hydnoceras,  345. 
Hydration,  23. 
Hydraulic  cement,  196,  205,  835. 

Icebergs,  108, 128. 

Ice  floes,  128. 

Ice  foot,  127, 128. 

Ichthyosaurus,  391. 

Idaho,  Shoshone  Falls  in,  73. 

Igneous  rocks,  206,  241. 

Illinois  (see  Chicago),  fossil  ferns  in, 

223. 

lead  ore  of,  236,  331. 

coal  in,  364. 

Indiana,  natural  gas  in,  320,  331. 
Inoceramus,  408. 
Insects,  329,  374,  390,  419. 
Interior,  of  earth,  289. 
Intrusive  sheet,  243. 
Iowa,  cross  bedding  in,  218. 

lake  ice  in,  113. 

weathered  rocks  in,  27. 
lowan  stage,  437. 
Ireland,  peat  in,  174. 
Iron,  Clinton,  340. 

compounds,  197. 

concentration  of,  175. 

ores  of,  239. 

pre-Paleozoic,  807. 
Iron  age,  460. 
Iroquois,  Lake,  442. 
Irrigation,  187. 
Isar  River,  63. 
Islands,  coral.  178. 
Isolelus,  329. 
Ithaca  formation,  844. 

Jackson  formation,  415. 
Japan,  earthquakes  in,  158,  159. 
Joints,  18,  230,  249. 
Jordan,  80. 


INDEX 


471 


Jupiter  Serapis,  Temple  of,  165. 

Libbey,  W.  F.,  265. 

Jura  Mountains,  259,  261,  379. 

Lignite,  196,  434. 

Jurassic  period,  378. 

Lignitic  beds,  414. 

Limestone,  18,  205. 

Kames,  266. 

calcium  caroonate  in,  196. 

Kansas,  fossil  reptiles  of,  409. 

lithographic,  397. 

glaciation  in,  437. 

metamorphic,  212. 

Kaolin,  204. 

origin  of,  175. 

Kentucky,  Mammoth  Cave  in,  87. 

Limonite,  198. 

Blue  Grass  Region  in,  332. 

Lingula,  298,  326,  348. 

Kettle-hole  ponds,  274. 

Lithographic  limestone,  397. 

Keweenaw  group,  312,  317. 

Lisbon  earthquake,  162. 

Kinderhook  group,  361. 

Loam,  204. 

Kitchen  middens,  460. 

Loess,  13. 

Kootauie  beds,  402. 

Long  Island,  dunes  of,  10. 

Krakatoa,  7,  151. 

Lorraine  beds,  321. 

Longitudinal  streams,  285. 

Laccolith,  243,  246. 

Lower  Silurian  period,  318. 

Lafayette  formation,  415. 

Lowville  limestone,  321, 

Lagoons,  180. 

Luray  Cavern,  87,  88. 

Lahontan,  Lake,  444. 

Luster,  193. 

Lakes,  110. 

Lycopods,  369. 

basins  of,  272,  274,  275. 

Lyell,  Sir  Charles,  161. 

beaver  ponds,  184. 

glacial,  444. 

Magnesian  limestone,  196. 

geological  work  of,  110. 

Magnesium,  192. 

plains  of,  252. 

Magnetite,  197. 

salt,  114. 

Maine,  raised  beaches  of,  165. 

sea  lochs,  166. 

Malaspina  Glacier,  106. 

sediments  of,  217. 

Mammals,  396,  420,  453. 

shores  of,  279. 

Mammoth,  456. 

Tertiary,  413,  415. 

Mammoth  Cave,  87. 

Lake  dwellings,  460. 

Man,  antiquity  of,  424,  457. 

Lamellibranchs,  350,  387,  419. 

geological  work  of,  185. 

Laminae,  216. 

Neolithic,  460. 

Landslides,  78,  159,  277. 

Paleolithic,  458. 

Lapilli,  141. 

Manlius  limestone,  336. 

Laramie  epoch,  400,  402. 

Marble,  212,  317,  331. 

Lavas,  138,  241-249. 

Marcellus  shale,  344. 

Lead,  236,  331. 

Marl,  204. 

Le  Conte,  80,  95,  143,  156,  256,  399. 

deep-sea,  131. 

Lehigh  River,  285. 

shell,  174,  176. 

Leperditia,  352. 

Marsh,  O.  C.,  409. 

Lepidodendron,  370,  371. 

Marshall  group,  361. 

Leptaena,  350. 

Martha's  Vineyard,  Cretaceous  in,  402, 

Lesley,  J.  P.,  361. 

404. 

Levees,  63,  183. 

dunes  on,  10. 

472 


GEOLOGY 


Martha's  Vineyard,  sand  blast  on,  5. 

shore  of,  125, 127. 

Tertiary  in,  414. 
Massachusetts,  Cambrian  in,  312. 

dikes  in,  249. 

bowlders  in,  429. 

drurnlins  in,  468,  469. 

granite  in,  207. 

lloosac  Tunnel  in,  186. 

lava  mountains  of,  246. 

mountains  of,  332. 

peneplain  in,  283. 

plateau  of  western,  253. 

ripple  marks  in,  220. 

sea  cliffs  of,  121-127. 

Somerville  slates  in,  230. 

Triassic  in,  380. 
Massive,  199. 
Mastodon,  421,  454. 
Maturity  of  land  forms,  282. 
Mauch  Chunk  shale,  361. 
McGee,  W  J,  62. 
Meanders,  46,  60,  63,  65. 
Medina  epoch,  334. 
Megatherium,  454. 
Merrill,  G.  P.,  21,  24,  32. 
Mesopithecus,  423. 
Mesozoic  era,  378. 
Metallic  elements,  192. 

deposits,  238. 
Metamorphism,  209. 
Mica,  195. 

Michigan,  copper  of,  239. 
Migration,  299,  453. 
Miller.  Hugh,  354. 
Minerals,  192. 
Mineralogy,  193. 
Mineral  springs,  80. 
Mining,  239. 
Miocene  epoch,  414. 
Missing  link,  459. 
Mississippi  River,  39,  51,  60,  63,  69,  74, 

167. 187,  272. 
Mohawk  River,  60. 

Valley,  284. 

faults,  238. 
Mollusks,  Carboniferous,  374. 


Mollusks,  Devonian,  350. 

Lower  Silurian,  326. 

Upper  Silurian,  339. 
Monkeys,  423. 
Monoclinal  fold,  227. 
Monograptus,  823. 
Montana  formation,  402. 
Monument  Park,  86. 
Moon,  craters  of,  147. 
Moraines,  98,  428. 
Mosasaur,  409. 
Mountains,  254. 

age  of,  257. 

Appalachian,  875. 

defined,  255. 

form  of,  257. 

height  of,  287. 

origin  of,  256. 
Mount  St.  Elias,  90,  91. 
Mud  cracks,  220. 
Murchison,  318,  334,  339. 
Murray,  Sir  J.,  131, 182. 
Muscovite,  195. 

Nantucket,  10, 166. 

Narragansett  Bay,  166. 

Natural  Bridge,  85,  86. 

Nautilus,  327. 

Nebular  hypothesis,  302. 

Neolithic  man,  460. 

Nevada,  81, 114,  289. 

Neve",  92. 

Newark  formation,  379. 

New  England,  earthquakes  in,  159. 

oscillations  in,  165. 166. 

peat  in,  174. 
New  Jersey,  Cretaceous  in,  402. 

dunes  in,  10. 

Highlands  of,  262. 

shore  of,  126. 

subsidence  in,  166. 

trap  sheet  in.  242,  246,  881. 

water  supply  in,  82,  83. 
New  York,  Ausable  Chasm  in.  43,  231, 
270. 

beaver  work  in,  184. 

Cambrian  in,  310,  312,  316. 


INDEX 


473 


New  York,  Catskill  plateau  in,  253. 

Ontario,  shore  of  Lake,  111. 

dike  in,  245. 

Oolitic,  200,  222. 

dip  of  rocks,  216. 

Ooze,  deep-sea,  131. 

drift  bowlders  in,  429. 
drumlins  in,  268. 

Ordovician  period,  318. 
Oregon,  altitude  of,  53. 

Finger  Lakes  in,  69. 

Crater  Lake  in,  276. 

glacial  lakes  in,  441,  442. 

lake  basins  in,  272. 

Howe's  Cave  in,  87. 

Ores,  240. 

Lower  Silurian  in,  392. 
mean  altitude  of,  53. 

Organisms,  work  of,  24,  170. 
Oriskany  epoch,  343. 

metamorphic  rocks  in,  210. 

Ormoceras,  327. 

Mohawk  Valley  in,  233,  284. 

Orthis,  326,  349. 

natural  gas  in,  332. 
Niagara  escarpment  in,  73. 

Orthoceras,  327,  351. 
Orthonota,  350. 

Palisades  of  Hudson,  244,  381. 

Orton,  E.,  331. 

peneplain  in,  283.   • 

Osage  group,  361. 

salt  in,  341. 

Oscillations,  163,  277. 

survey  of,  318. 

Osteolepis,  353. 

Trenton  gorge  in,  41,  72,  270. 

Outcrop,  228. 

Niagara  epoch,  335. 

Oxidation,  22,  76. 

Niagara,  age  of,  451. 

Oxygen,  192. 

diversion  of,  168. 

Oyster,  296,  387,  419. 

escarpment,  73. 

Falls,  73. 

Paleolithic  man,  458. 

history  of,  442. 

Paleontology,  292. 

Nile,  51,  60,  66,  69,  74,  187. 

Paleozoic  era,  308. 

Normal  fault,  234. 

geography  of,  308. 

Norway,  fiords  of,  1(J6. 

life  of,  309. 

Nova  Scotia,  coal  in,  366. 

Palisades    of   the    Hudson,    56,    244, 

381. 

Obsidian,  139,  207,  208. 

Paradoxides,  315. 

Ocean,  116. 

Peat,  173,  196. 

basins,  288. 

Pecten,  387. 

currents,  118. 

Peneplain,  283. 

deposits  of,  131. 

Cretaceous,  401. 

depths  of,  130,  132. 

Pennsylvania,  Carboniferous  in,  361, 

Ohio,  gas  of,  320,  331. 

364. 

glacial  deposits  of,  427. 

coal  and  iron  in,  175. 

Hudson  rocks  in,  321. 

Medina  in,  334. 

salt  in,  340. 

mountains  of,  227,  257. 

Oil,  in  shales,  204. 

peneplain  in,  283,401. 

sands,  357. 

rivers  of,  285. 

Old  Red  Sandstone,  355. 

solution  in,  21. 

Olenellus,  314. 

spring  in,  80. 

Oneida  Conglomerate,  334. 

topography  of,  34. 

Oneonta  formation,  344. 

Trenton  in,  320. 

Onondaga  epoch,  343. 

Pentamerus,  337,  339,  349. 

474 


GEOLOGY 


Permian  period,  860,  867,  872. 

Petrography,  199. 

Petroleum,  357. 

Pisolitic,  200. 

Phacops,  352. 

Phosphatic  rocks,  185,  424. 

Physiographic  structures,  250. 

Placers,  240,  397. 

Placodernis,  363. 

Plains,  252. 

Plants,  Carboniferous,  368. 

Cretaceous,  404. 

geological  work  of,  170. 

Lower  Silurian,  330. 

Triassic  and  Jurassic,  383. 

Tertiary,  417. 
Plaster  of  Paris,  197. 
Plateaus,  252. 
Pleistocene  period,  426. 
Plesiosaurus,  392. 
Pleurotomaria,  327,  328,  339. 
Pliocene  epoch,  414. 
Plutonic  rocks,  206,  207. 
Po,  54,  64,  69. 
Pocono  sandstone,  361. 
Porphyritic,  200. 
Portage  formation,  344. 
Potassium,  192. 
Potholes,  84. 
Potomac,  70. 
Potsdsm  epoch,  310. 
Pottsville  Conglomerate,  361. 
Powell,  J.  W.,  260. 
Prestwich,  12. 
Primates,  423. 
Proboscidians,  421. 
Productella,  349. 
Productus,  374. 
Protozoa,  313,  372,  406. 
Protypus,  315. 
Ptericthys,  353. 
Pterinea,  350. 
Pterodactyl,  395. 
Pteropod,  328,  339. 
Pterosaurs,  395. 
Pumice  stone,  145,  208. 
Pynchon,  W.  H.  C.,  221 


Pyrenees,  417. 
Pyrites,  198. 

Quartz,  194. 
Quartzite,  218. 
Quaternary  period,  426. 
Quicklime,  205,  331. 

Radiolaria,  385. 
Kain,  erosion  by,  18. 

formation  of,  37. 

rainfall  in  the  United  States,  38. 

run-otf,  38. 
Rapids,  71. 
Kcd  ochre,  197. 
Red  River,  51,  173,  284. 
Reefs,  coral,  179. 
Rensselaeria,  350. 
Reptiles,  375,  378,  391,  409. 
Reversed  fault,  234. 
Rhine,  51,  60,  69,  70, 142,  271. 
Rhizocarp,  331. 
Rhode  Island,  coal  in,  366. 
Rhone  Glacier,  96. 

Rhone  River,  51,  54,  62,  69, 70, 187,  280. 
Rhynchotrema,  326. 
Rhyolite,  208. 
Richmond  earth,  424. 
Rill  marks,  219. 
Ripley  group,  402. 
Ripple  marks,  219,  316. 
Rivers,  37. 

bars,  71. 

classification  of,  284. 

climate  and,  74. 

deltas,  66. 

deposition  by,  54. 

discharge  of  solids,  51. 

down-cutting  by,  44. 

economic  significance  of,  74. 

erosion  by,  41. 

estuaries,  70. 

flood  plains  of,  58. 

meanders  of,  60. 

overloaded,  42. 

parts  of,  89. 

plains  made  by,  252. 


INDEX 


475 


Rivers,  rapids,  71. 

terraces,  64. 

transportation  by,  48. 

waterfalls,  71. 

Roches  moutonnees,  97,  445. 
Rocks,  17. 

color  of,  28, 175,  201,  205. 

composition  of,  199. 

coral,  179. 

fraginental,  201,  215. 

gross  structure  of,  215. 

igneous,  206,  241. 

induration  of,  77. 

metamorphic,  209,  212. 
Rock  basins,  275. 
Rock-forming  minerals,  191. 
Rock  salt,  115,  340,  397. 
Rocky  Mountains,  109,  256,  305,  377, 

400. 

Rogers,  H.  D.  and  W.  B.,  361. 
Rooting  slate,  213. 
Ruedemann,  322. 
Ruasell,  1.  C.,  6,  53,  65, 106,  272,  879. 

Salina  epoch,  335. 
Salisbury  Crags,  246. 
Salisbury,  R.  D.,  448. 
Salt,  common,  197,  340,  397. 
Salt  lakes,  114. 
Sand,  202. 

blast,  5. 

coral,  179. 

plain,  glacial,  445. 

stone,  18,  202,  317. 

storms,  6. 

Saratoga,  springs  of,  80. 
Satin  spar,  197. 
Schistose  structure,  200,  213. 
Schuylkill  River,  285. 
Schuchert,  C..  321. 
Scorpion,  fossil,  339,  340. 
Scott,  W.  B.,  228,  894,  416,  421. 
Scotland,  drumlinoid  forms  in,  269. 

fiords  of,  166. 

Highlands  of,  262. 

lakes  of,  276. 

marine  erosion  in,  119, 124. 


Scotland,  Old  Red  Sandstone  in,  355. 

overthrust  in,  235. 

peneplain  in,  283. 

volcanic  rocks  in,  141,  246,  247. 
Sea  urchins,  Mesozoic,  386. 
Sedgwick,  A.,  310. 
Sedimentary  rocks,  201,  215. 
Seine  River,  62. 
Selachians,  354. 
Selenite,  197. 
Septaria,  223. 
Sequoia,  404. 
Serpentine,  195. 
Shale,  18,  200,  204. 
Shale,  Utica,  321. 
Shaler,  N.  S.,  65,  84, 125,  159,  174,  249, 

436,  452. 

Shasta,  Mount,  264. 
Shawangunk  Grit,  334. 
Sheet,  volcanic,  243,  244. 
|  Shore  lines,  277,  430. 
j  Sierras,  255,  377,  383. 
Sigillaria,  369,  371. 
Silica,  194. 
Silicates,  194. 
Silicified  wood,  174. 
Silicon,  192. 

Silurian  periods,  318,334. 
Skye,  torrent  in,  50. 
Slate,  212. 
Soapstone,  195. 
Sodium,  192. 
Sodium  chloride,  197. 
Soils,  29,  332,  452. 
Solution,  20,  77,  94. 
Spencer,  J.  W.,  442. 
Sphagnum,  173. 
Spirifer,  298,  337,  349,  373. 
Sponges,  345,  406. 
Springs,  79, 186. 

mineral,  80. 

thermal,  81. 

St.  Lawrence  River,  41, 110. 
St.  Louis  group,  361. 
Stalactite,  85. 
Stalagmite,  85. 
Starfishes,  347. 


476 


GEOLOGY 


Steatite,  195. 

Tides,  116. 

Stegosaurus,  394. 

Till,  428. 

Streak,  193. 

Time,  geological,  460. 

Stratification,  18,  215. 

Tin,  240,  307. 

Strife,  glacial,  96,  97,  431. 

Tombigbee  sands,  402. 

Strike,  228. 

Trachyte,  208. 

Structural  geology,  1,  191. 

Tracks,  fossil,  316,  317. 

Subsequent  streams,  284. 

Transportation,  4. 

Subsidence,  of  shores,  277. 

by  glaciers,  97. 

proofs  of,  165. 

by  rivers,  48. 

regions  of,  1  66. 

Transverse  streams,  285. 

theorj  of  coral  reefs,  180. 

Trap  rock,  208. 

Subterranean  waters,  76. 

Travertine,  81,  205. 

Suess,  E.,  451. 

Trellised  drainage,  285. 

Sulphates,  197. 

Trenton,  epoch,  320,  331. 

Sun  cracks,  220. 

gorge,  41,  72,  318. 

Susquehanna  Kiver,  41,  60,  70,  284,  285. 

Triiussie  period,  378. 

Sweden,  subsidence  in,  167. 

Trigonia,  387. 

Switzerland  (see  Alps),  alluvial  cone 

Tropidoleptus,  849. 

in,  57. 

Triarthrus,  328,  880. 

diversion  of  river  in,  187. 

Trilobitcs,  296. 

erosion  in,  282. 

Carboniferous,  374. 

glaciation  of,  432. 

Cambrian,  314. 

glaciers  of,  98-106. 

Devonian,  352. 

lakes  of,  276. 

Lower  Silurian,  328. 

landslides  in,  78,  79. 

tracks  of,  31  6. 

torrents  in,  40. 

Upper  Silurian,  339. 

Syenite,  208. 

Trinucleus,  328,  329. 

Synclinal  fold,  225. 

Tufa,  81,  142,  205. 

Turrilites,  408. 

Table  mountains,  255. 

Tuscaloosa  group,  402. 

Talc,  195. 

Tyndall,  93. 

Talus,  55. 

Taylor,  F.  B.,  442. 

Udden,  J.  A.,  6. 

Tejon  series,  415. 

Uinta  Mountains.  259,  260. 

Tennessee,  overthrust  in,  235. 

Unconformity,  228. 

Tentaculite,  337. 

Underground  water,  76. 

Tentuculite  limestone,  336. 

Upham,  W.,  440.  461. 

Terraces,  64. 

Upper  Silurian  period,  834. 

marine,  165. 

Utah,  shore  lines  in,  279. 

Tertiary  period,  411. 

Utica  epoch,  321. 

Texas,  63,  205. 

Thames  Kiver,  51,  60,  69. 

Valleys,  45,  270. 

Thermal  springs,  81. 

Veins,  236,  397. 

Thoreau,  II.  D.,  10. 

Verd-antique,  195. 

Throw,  of  fault,  232. 

Vermont,  Cambrian  in,  811. 

Thrust  fault,  234. 

Green  Mountains  in,  256,  832. 

INDEX 


477 


Vermont,  marble  in,  210. 

veins  in,  237. 
Vesuvius,  134, 140. 
Vicksburg  formation,  415. 
Viesch  Glacier,  frontispiece,  94. 
Virginia,  James  River  in,  65. 

Luray  Cavern  in,  87,  88. 

Natural  Bridge  in,  85,  86. 

Richmond  coal  in,  397. 

Richmond  earth  in,  424. 
Vitreous  rocks,  200. 
Volcanic  cone  and  neck,  243. 

rooks,  208. 

topography,  262. 
Volcanoes,  134. 

active,  dormant,  extinct,  137. 

American  Tertiary,  413. 

causes  of,  152. 

decadent,  143. 

distribution  of,  145. 

dust  from,  7. 

fragrnental  products  of,  141. 

gases  from,  137. 

principles  of,  137. 

Walcott,  G.  D.,  313,  315,  317,  329,  461. 

Wallace,  A.  R.,  4G1. 

Warping,  167. 

Warren,  Lake,  441. 

Wasatch  Mountains,  400. 

Water,  city  supply  of,  22,  82. 

in  all  rocks,  76. 

in  volcanic  eruptions,  138. 

power,  71,  74,  452. 

underground,  76. 

weathering  agent,  18. 


Waterfalls,  71. 
Waterlirne  group,  335. 
Waverley  group,  361. 
Waves,  earthquake,  156. 

ocean,  118,  158. 
Weathering,  16. 

agents  of,  18. 

dependent  on  climate,  28. 

effects  of,  28. 
Wells,  82. 

artesian,  83. 
Whales,  Tertiary,  420. 
White  Mountains,  avalanches  in,  109. 

landslide  in,  79. 
Williams,  G.  H.,  223. 
Willis,  B.,  224. 
Winds,  3,  4,  6,  8, 10,  277. 
Wisconsin,  driftless  area  in,  446. 

drumlins  in,  268. 

weathering  in,  36. 
Wisconsin  stage,  437. 
Wright,  G.  F.,  427. 
Wyoming,  fossil  reptiles  of,  393,  394. 

Yellow  ochre,  198. 

Yellowstone  Tark,  geysers  of,  143. 

hot  springs  in,  81,  82. 

lavas  of.  139. 

Obsidian  Clitf  in,  208. 

weathering  in,  24. 
Yoscmite.  74. 
Yukon,  49,  69. 

Zaphrentis,  346. 
Zeuglodon,  420. 


(8) 


THE   END 


TWENTIETH  CENTURY  TEXT-BOOKS. 


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